feat(git): add local branch workflows to new sessions (#376)

## Summary

- add searchable branch selection and quick branch creation to the
new-session flow
- make branch choice independent from Local / Worktree execution mode
- safely preflight Local branch switches, show overlapping changed files
and line statistics, and support commit-and-continue
- keep Worktree mode isolated from the original checkout and add wire,
SDK, engine, mock, cache invalidation, and i18n support
- keep the branch search icon and text in separate `InputGroup` layout
slots so they cannot overlap

Linear:
[CODE-527](https://linear.app/arcbox/issue/CODE-527/featgitui-add-local-branch-workflows-to-new-sessions)

## Verification

- `pnpm check:ci`
- `pnpm test` — 300 test files / 2408 tests passed; 1 file / 1 test
skipped
- pre-commit hooks
- `git diff --check`
- observed the complete branch search, creation, Local / Worktree,
conflict, and commit-and-continue flow in the mock webview
- visually rechecked the search field after merging current `master`;
icon and placeholder have a measured 4px gap with no clipping or overlap

## Checklist

- [x] `pnpm check:ci` and `pnpm test` both pass (plus `cargo fmt` /
`clippy` / `test` for Rust changes)
- [x] I ran the affected surface and observed the change working
- [x] If a wire message changed: `WIRE_PROTOCOL_VERSION` is bumped
- [x] New code and assets are my own work, or their origin and license
compatibility are noted above
- [x] Docs and comments are updated where behavior changed

Author: AprilNEA

packages/client/core/src/client.ts

@@ -18,6 +18,7 @@ import type {
   EffortLevel,
   FileSuggestion,
   GitBranchList,
+  GitBranchSwitchCheck,
   GitDiff,
   GitDiffMode,
   GitPullRequestStatus,
@@ -509,6 +510,9 @@ export class LinkCodeClient {
       case 'git.branch.list.result':
         this.pending.resolve('gitBranchList', p.replyTo, p.branchList);
         break;
+      case 'git.branch.switch.check.result':
+        this.pending.resolve('gitBranchSwitchCheck', p.replyTo, p.check);
+        break;
       case 'git.pr_status.get.result':
         this.pending.resolve('gitPrStatus', p.replyTo, p.prStatus);
         break;
@@ -1132,6 +1136,18 @@ export class LinkCodeClient {
     return this.control.listGitBranches(cwd);
   }
 
+  checkGitBranchSwitch(cwd: string, branch: string): Promise<GitBranchSwitchCheck> {
+    return this.control.checkGitBranchSwitch(cwd, branch);
+  }
+
+  createGitBranch(cwd: string, branch: string): Promise<RequestAck> {
+    return this.control.createGitBranch(cwd, branch);
+  }
+
+  commitGitChanges(cwd: string, message: string): Promise<RequestAck> {
+    return this.control.commitGitChanges(cwd, message);
+  }
+
   getGitPullRequestStatus(cwd: string): Promise<GitPullRequestStatus> {
     return this.control.getGitPullRequestStatus(cwd);
   }

packages/client/core/src/client/control-channel.ts

@@ -19,6 +19,7 @@ import type {
   EffortLevel,
   FileSuggestion,
   GitBranchList,
+  GitBranchSwitchCheck,
   GitDiff,
   GitDiffMode,
   GitPullRequestStatus,
@@ -598,6 +599,33 @@ export class ControlChannel {
     }));
   }
 
+  checkGitBranchSwitch(cwd: string, branch: string): Promise<GitBranchSwitchCheck> {
+    return this.sendCorrelated('gitBranchSwitchCheck', (clientReqId) => ({
+      kind: 'git.branch.switch.check',
+      clientReqId,
+      cwd,
+      branch,
+    }));
+  }
+
+  createGitBranch(cwd: string, branch: string): Promise<RequestAck> {
+    return this.sendCorrelated('ack', (clientReqId) => ({
+      kind: 'git.branch.create',
+      clientReqId,
+      cwd,
+      branch,
+    }));
+  }
+
+  commitGitChanges(cwd: string, message: string): Promise<RequestAck> {
+    return this.sendCorrelated('ack', (clientReqId) => ({
+      kind: 'git.commit',
+      clientReqId,
+      cwd,
+      message,
+    }));
+  }
+
   /** Hosting-provider PR state for a directory's current branch. */
   getGitPullRequestStatus(cwd: string): Promise<GitPullRequestStatus> {
     return this.sendCorrelated('gitPrStatus', (clientReqId) => ({

packages/client/core/src/client/pending-registry.ts

@@ -8,6 +8,7 @@ import type {
   CustomMcpServerPublic,
   FileSuggestion,
   GitBranchList,
+  GitBranchSwitchCheck,
   GitDiff,
   GitPullRequestStatus,
   GitStatus,
@@ -111,6 +112,7 @@ export interface PendingValueMap {
   assetEnsure: ManagedAssetStatus;
   gitStatus: GitStatus;
   gitBranchList: GitBranchList;
+  gitBranchSwitchCheck: GitBranchSwitchCheck;
   gitPrStatus: GitPullRequestStatus;
   gitDiff: GitDiff;
   fileRead: WorkspaceFile;
@@ -172,6 +174,7 @@ export class PendingRegistry {
     assetEnsure: new Map(),
     gitStatus: new Map(),
     gitBranchList: new Map(),
+    gitBranchSwitchCheck: new Map(),
     gitPrStatus: new Map(),
     gitDiff: new Map(),
     fileRead: new Map(),

packages/client/core/tests/integration/control-client.test.ts

@@ -16,6 +16,35 @@ import { createConnectedLocalClient } from '../support/local-client';
 const sessionId = 'sess-control' as SessionId;
 
 describe('LinkCodeClient control API', () => {
+  it('routes git branch switch checks and mutation acknowledgements', async () => {
+    const { client, serverTransport } = await createConnectedLocalClient();
+    serverTransport.onMessage((msg) => {
+      const p = msg.payload;
+      if (p.kind === 'git.branch.switch.check') {
+        serverTransport.send(
+          createWireMessage({
+            kind: 'git.branch.switch.check.result',
+            replyTo: p.clientReqId,
+            check: { status: 'ready' },
+          }),
+        );
+      } else if (p.kind === 'git.branch.create' || p.kind === 'git.commit') {
+        serverTransport.send(
+          createWireMessage({ kind: 'request.succeeded', replyTo: p.clientReqId }),
+        );
+      }
+    });
+
+    await expect(client.checkGitBranchSwitch('/repo', 'main')).resolves.toEqual({
+      status: 'ready',
+    });
+    await expect(client.createGitBranch('/repo', 'feature')).resolves.toEqual({ ok: true });
+    await expect(client.commitGitChanges('/repo', 'save')).resolves.toEqual({ ok: true });
+
+    client.dispose();
+    serverTransport.close();
+  });
+
   it('preserves MCP warnings on detailed session-start results', async () => {
     const { client, serverTransport } = await createConnectedLocalClient();
     serverTransport.onMessage((msg) => {

packages/client/sdk/src/client.ts

@@ -26,6 +26,7 @@ import type {
   EffortLevel,
   FileSuggestion,
   GitBranchList,
+  GitBranchSwitchCheck,
   GitDiff,
   GitDiffMode,
   GitPullRequestStatus,
@@ -380,6 +381,18 @@ export class LinkCodeSdkClient {
     return toResult(this.raw.listGitBranches(cwd));
   }
 
+  checkGitBranchSwitch(cwd: string, branch: string): RequestResult<GitBranchSwitchCheck> {
+    return toResult(this.raw.checkGitBranchSwitch(cwd, branch));
+  }
+
+  createGitBranch(cwd: string, branch: string): RequestResult<{ ok: true }> {
+    return toResult(this.raw.createGitBranch(cwd, branch));
+  }
+
+  commitGitChanges(cwd: string, message: string): RequestResult<{ ok: true }> {
+    return toResult(this.raw.commitGitChanges(cwd, message));
+  }
+
   /** Hosting-provider PR state for a directory's current branch. */
   getGitPullRequestStatus(cwd: string): RequestResult<GitPullRequestStatus> {
     return toResult(this.raw.getGitPullRequestStatus(cwd));

packages/client/sdk/src/operations.ts

@@ -23,6 +23,7 @@ import type {
   EffortLevel,
   FileSuggestion,
   GitBranchList,
+  GitBranchSwitchCheck,
   GitDiff,
   GitDiffMode,
   GitPullRequestStatus,
@@ -353,6 +354,24 @@ export function listGitBranches(options: Options<{ cwd: string }>): RequestResul
   return resolveClient(options).listGitBranches(options.cwd);
 }
 
+export function checkGitBranchSwitch(
+  options: Options<{ cwd: string; branch: string }>,
+): RequestResult<GitBranchSwitchCheck> {
+  return resolveClient(options).checkGitBranchSwitch(options.cwd, options.branch);
+}
+
+export function createGitBranch(
+  options: Options<{ cwd: string; branch: string }>,
+): RequestResult<{ ok: true }> {
+  return resolveClient(options).createGitBranch(options.cwd, options.branch);
+}
+
+export function commitGitChanges(
+  options: Options<{ cwd: string; message: string }>,
+): RequestResult<{ ok: true }> {
+  return resolveClient(options).commitGitChanges(options.cwd, options.message);
+}
+
 /** Hosting-provider PR state for a directory's current branch. */
 export function getGitPullRequestStatus(
   options: Options<{ cwd: string }>,