feat: projection slot V1 — cross-agent config sync (#31)

[tu11aa]

Summary

Ships cockpit projection — emits cockpit's canonical content (skills at user level, AGENTS.md at project level) to each supported agent's expected path. Completes the multi-agent direction from 50c63b3 by fixing the "missing many things" experience when opening cockpit-managed projects in Cursor/Codex/Gemini CLI.

Closes #31. Follow-ups: #34 (MCP sync), #35 (role identity).

What's in V1

• Two-tier projection: user-level (from plugin/skills/*/SKILL.md) vs project-level (from {project}/AGENTS.md)
• Three targets: Cursor (.cursor/rules/*.mdc), Codex (AGENTS.md), Gemini CLI (GEMINI.md)
• Marker-merge for shared files (<!-- cockpit:start --> ... <!-- cockpit:end -->), overwrite for dedicated files
• CLI: emit, diff (dry-run), list; flags: --scope, --project, --target, --all
• Doctor probe for destination writability
• Argument validation: unknown project/target, mutually exclusive flags (--scope user + --project, --all + others)
• README + Supported Agents table updated

Design & Plan

• Spec: docs/specs/2026-04-24-plugin-system-projection-design.md
• Plan: docs/specs/2026-04-24-plugin-system-projection-plan.md

Test plan

• All unit tests pass — marker (8), canonical-source (7), cursor (7), codex (7), gemini (6), registry (3), CLI (8) = 46 new tests
• npx tsc --noEmit clean
• Full suite: 171 passing, 2 pre-existing config.test.ts emoji failures (unrelated)
• npm run build succeeds
• Smoke: projection list shows three targets with correct paths
• Smoke: projection diff --scope user --target cursor previews NEW FILE with frontmatter + inlined skill content
• Smoke: cockpit doctor shows Projection section with per-target status

Architecture mirrors prior plugin phases

Same driver+registry+CLI pattern as runtime/workspace/tracker/notifier. Each emitter implements the ProjectionEmitter interface. The marker-merge helper is shared across shared-file emitters (codex, gemini). Cursor emitter overwrites dedicated .mdc files.

Known limitations (follow-ups)

• User-level projection assumes cwd is the cockpit repo (where plugin/skills/ lives). Running from outside the repo emits an empty user-level source. Worth revisiting when Karpathy-principles etc. want to project from an installed location rather than the source checkout.
• No shared render helper between codex.ts and gemini.ts yet — intentional per plan; extract when a 3rd markdown emitter lands.
• MCP config sync tracked in projection: MCP config sync across agents (follow-up to #31) #34.
• Role identity (captain/crew for non-Claude agents) tracked in projection: captain/crew role identity for non-Claude agents (follow-up to #31) #35.