build: extract @copse/plugin-sdk, the plugin runtime SDK and host protocol

[jonathanKingston]

Summary

• Adds packages/plugin-sdk (@copse/plugin-sdk): plugin-tool-protocol, plugin-tool-sdk, plugin-tool-worker, plugin-tool-source, plugin-tool-snapshot, plugin-browser-service, and plugin-model-turn from src/main/services/plugins/; mcp-schema and mcp-config from src/main/services/mcp/; and the MCP configuration types from src/shared/types/mcp.ts. About 1,800 LOC plus tests. Depends on @copse/agent, @copse/llm, @copse/std, zod; no host-app imports.
• Introduces configurePluginSdk for the one host fact the snapshot helper needs, the profile root reviewed snapshots live under (default COPSE_DIR or ~/.copse). The app binds copseDataRoot in src/main/services/plugins/plugin-sdk-environment.ts, imported for its side effect by the app-side re-exports.
• The standalone worker bundle keeps its entry path. src/main/services/plugins/plugin-tool-worker.ts is now a one-line entry importing the package worker, so scripts/main-bundles.mts, the stdout-protocol guard's entry list, and the host's join(__dirname, 'plugin-tool-worker.js') lookup are unchanged.
• Every previous import path is a re-export of the package; the plugin-tool-sdk stub had no remaining importers and was deleted. custom-tools-config stays in the app since it depends on the app's ToolDefinition contract; the registry service, sandboxed tool host and controller, disk discovery, browser panel, MCP registry, and curated list stay as the host.
• docs/plugins.md's module layout names the package; package README added.

Closes #2309. Step 6 of the library-splits plan (#2303). Branches from main; all four earlier extractions are already merged.

Behaviour is unchanged

No protocol message, schema, or snapshot rule changed. The moved tests (plugin-tool-sdk, plugin-tool-source, plugin-tool-snapshot, plugin-model-turn, mcp-schema, mcp-config) pass unmodified inside the package; the host tests (plugin-tool-host, plugin-tool-controller, plugin-service, mcp-registry-persistence, custom-tools-registry, plugin-browser-panel) pass unmodified through the re-exports.

Validation

• pnpm test -- workspace-package-invariants pnpm-workspace-invariants module-boundaries stdout-protocol-guard settings-read-keys packages/plugin-sdk/ src/main/services/plugins/ src/main/services/mcp/ permission-gate — 283 pass
• pnpm test (full unit suite) — 8,253 pass
• pnpm run typecheck, pnpm run lint, pnpm run check:dead-code, pnpm run build — clean
• Build plumbing and an equal-behaviour move; no user-visible change, so no visual evidence is required.

🤖 Generated with Claude Code

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}

[copse-release-bot]

Reference screenshots ready for review

Review GitHub’s image diffs in screenshot PR #2336.
Merge it (or enable auto-merge) to apply the accepted PNGs to claude/extract-plugin-sdk.

Rendered for 71c068f63e93 by CI run 33816195270; the immutable artifact is reference-screenshot-candidates-33816195270.

If this source branch moves, a later successful render closes the stale review PR and replaces this link.