Add Pane agent context discovery

[parsakhaz]

Summary

• Adds runpane agent-context for brief agent-facing Pane command discovery and lazy detailed command definitions via --command <name>.
• Extends the shared runpane contract/schema and regenerates Node, Python, shared, fixture, and docs artifacts.
• Adds default-on managed AGENTS.md support with a Settings toggle, best-effort project lifecycle wiring, and removal of Pane-owned blocks when disabled.

Closes #257

Validation

• node scripts/generate-runpane-contract.js --check
• pnpm run test:runpane-contract
• pnpm --filter main exec vitest run src/services/agentContextManager.test.ts src/ipc/runpane.test.ts
• PYTHONDONTWRITEBYTECODE=1 python3 -B -m compileall -q packages/runpane-py/src/runpane
• pnpm typecheck
• pnpm lint (passes with existing warnings)
• Smoke: node packages/runpane/dist/cli.js agent-context
• Smoke: node packages/runpane/dist/cli.js agent-context --command "panes create" --json
• Smoke: PYTHONPATH=packages/runpane-py/src python3 -m runpane agent-context
• Smoke: PYTHONPATH=packages/runpane-py/src python3 -m runpane agent-context --command "panes create" --json

Notes

• Local environment prints the repo's Node engine warning because this shell is on Node v20.19.3 while the repo asks for Node >=22.14.0; validation still passed.

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[parsakhaz]

PR Review

Issue context: #257 - Add lazy-loadable Pane agent context and managed AGENTS.md section

Quality Gates

• Typecheck: PASS
• Lint: PASS (0 errors, 165 warnings - all pre-existing)

Must-Fix (0)

None.

Should-Fix (0)

None.

Suggestions (2)

1. Extra scope: autoStartOnBoot setting and startPinned default - The PR adds an autoStartOnBoot toggle to Settings.tsx and configManager.ts, plus a startPinned: false default in getSessionCreationPreferences(). Neither is related to issue #257 (agent context). Both are clean and harmless, but splitting them into their own commit or PR would keep the diff focused on one concern.

2. Empty AGENTS.md left behind when disabling (agentContextManager.ts:84) - When the managed block is the only content in AGENTS.md, disabling the setting writes an empty string rather than deleting the file. This is safe and the test explicitly covers it, but an empty AGENTS.md file in a repo could confuse users. Consider deleting the file when next.trim().length === 0 and Pane was the one that created it. Only a suggestion since it's a minor UX edge case and the current behavior is explicitly tested.

Completeness

The implementation covers all acceptance criteria from issue #257 and the follow-up decision comments:

• runpane agent-context prints a brief, token-efficient view (contract-driven) - done
• runpane agent-context --command <name> prints detailed definitions - done
• --json support for machine-readable output - done
• Managed AGENTS.md block with delimited markers, idempotent upsert, content-preserving insert/replace/remove - done
• Settings toggle (default-on) with disable-removes-from-all-projects behavior - done
• Lifecycle wiring: project create, project activate, config toggle change - done
• Node and Python implementations mirror each other from the shared contract - done
• Tests: agentContextManager unit tests (create, update, replace, remove, symlink safety), config IPC handler test (disable removes from all projects), runpane IPC handler tests (repos list/add/create, dry-run, idempotent add) - done
• Schema validation, fixture tests, and --check mode in the generator - done

Summary

This is a well-crafted PR. The architecture is clean: a single contract.json drives all generated surfaces (Node, Python, shared types, docs, fixtures), the agentContextManager has careful symlink protection via O_NOFOLLOW, idempotent block management with content comparison to avoid unnecessary writes, and all IPC integration points use best-effort error handling so a failed AGENTS.md write never blocks project creation or activation. Test coverage is solid across the manager, the config handler, and the runpane IPC handlers. Ready to merge.