fix: remove inert project-level GitHub identity override endpoints/UI (keep DB store)

[sabbour]

Summary

PR 2 from Link's investigation of PR #934: removes the now-inert project-level GitHub identity
override endpoints, service, and frontend calls left behind after #934 removed the runtime
override behavior from CallerTokenScopeProvider/ProjectAuthorization.

Scope update (2026-08-25): a design decision is in progress to repurpose the
project_github_identity_overrides table as a "workflow Copilot owner" store — GitHub App
installation tokens have no Copilot entitlements, so automation runs (scheduled/webhook) still
need a human user's Copilot-entitled token for inference. Per that decision, this PR no longer
deletes the DB table, the ProjectGitHubIdentityOverrideRecord entity, the
ProjectGitHubIdentityOverrideStore, or its DI registration — those are kept intact for future
reuse. Only the inert HTTP endpoints/service and the dead frontend API calls/types are removed.

Backend

• Deleted Auth/ProjectGitHubIdentityService.cs and Endpoints/ProjectGitHubIdentityEndpoints.cs.
• Removed the ProjectGitHubIdentityService DI registration and the
  MapProjectGitHubIdentityEndpoints() call from Program.cs.
  ProjectGitHubIdentityOverrideStore's DI registration is kept.
• Removed the overrideStore.RemoveOverridesForLinkedLoginAsync(...) call and DI injection from
  LinkedGitHubAccountService.cs (that cleanup was tied to the removed per-request override path).
• Removed the now-dead UpdateProjectGitHubIdentityRequest/ProjectGitHubIdentityResponse DTOs
  (they only existed to serve the deleted endpoints).
• Updated docs/reference/api.md to drop the removed endpoint rows.
• Kept intact: Auth/ProjectGitHubIdentityOverrideStore.cs,
  apps/Agentweaver.Api.Data/Memory/ProjectGitHubIdentityOverrideRecord.cs, the DbSet/model
  config in MemoryDbContext.cs, and the project_github_identity_overrides table (no migration
  to drop it in this PR).

Frontend

• GitHubSignIn.tsx: the "switch account" UI is unchanged. handleSwitch now unconditionally
  calls setDefaultLinkedGitHubAccount(login) instead of branching on a project-level override;
  removed projectIdentity state and the getProjectGitHubIdentity fetch; currentAccount now
  derives only from linkedAccounts.find(a => a.is_default).
• ProjectSettingsPage.tsx: removed the "GitHub identity for this project" section,
  handleSaveGitHubIdentityOverride, and related state/effects. General access/role UI is
  unchanged.
• api/client.ts / api/types.ts: removed getProjectGitHubIdentity,
  setProjectGitHubIdentityOverride, and the ProjectGitHubIdentity type (unused after the above).

Tests

• LinkedGitHubAccountsApiTests.cs: removed tests exercising the deleted
  /api/projects/{id}/github-identity endpoints (5 tests). Endpoint-independent tests
  (LinkCallback_..., AccessibleRepos_..., UnlinkDefaultAccount_...) are untouched.
• client.test.ts, GitHubSignIn.test.tsx, ProjectSettingsPage.test.tsx: updated to match the
  frontend changes above; GitHubSignIn.test.tsx now asserts setDefaultLinkedGitHubAccount is
  called when switching accounts.

Verification

• dotnet build on Agentweaver.Api, Agentweaver.Tests, Agentweaver.Api.Migrations.Postgres — 0 errors.
• dotnet test --filter "FullyQualifiedName~LinkedGitHubAccountsApiTests|FullyQualifiedName~CallerTokenScopeProviderTests" — 6/6 passed.
• npm --prefix apps/web run lint — clean.
• npx vitest run on the 3 affected frontend test files — 27/27 passed.

Changeset

.changeset/remove-project-github-identity-override.md (patch), updated to reflect the reduced scope.

Not merged — awaiting review, per instructions.

[sabbour]

Closing as superseded by the Fleet cutover. #946 removes the relevant Project Settings controls, and #951 will remove the remaining legacy identity path.