Move project instruction policy into plugins

[jonathanKingston]

Summary

• add stable first-party copse.claude-md, copse.agents-md, and copse.cursor-rules instruction-source plugins
• keep filesystem reads, workspace trust, realpath containment, byte/file caps, activation, deduplication, and prompt rendering in the host instruction engine
• implement the four Claude-compatible instruction modes, including per-directory nested Claude/Agents selection
• migrate existing profiles to combined behavior while fresh profiles use Claude-or-Agents fallback
• expose instruction-source contributions in Settings and reject them from user plugins
• document the privileged boundary and add focused unit, migration, registry, UI, and visual e2e coverage

This PR is stacked on #2701 so it contains only the follow-on instruction-source work.

Verification

• pnpm run typecheck
• pnpm run lint
• pnpm run format:check
• pnpm run check:dead-code
• pnpm run check:oracle
• pnpm run check:e2e-syntax
• pnpm run demo:site:check
• pnpm run build
• focused unit/component suite: 143 passing
• pnpm run test:e2e -- --spec tests/e2e/settings-plugins.e2e.ts: 1 passing

The unrestricted full test command could not complete in the filesystem sandbox because unrelated socket/loopback suites fail with EPERM; the focused Electron e2e passed outside that network sandbox.

Follow-ups

Full Claude parity still needs import expansion and a per-agent/fork-inherited instruction delivery ledger.

[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
• Unattended container run
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• 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
• Apple Development test profile
• 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

CI run 35532176743 published no changed screenshot candidates for a3d5d7c71f32.
Any review PR for an older source SHA has been closed.