feat(mcpd+mcplocal+cli): propose-learnings system skill, propose_skill MCP tool, mcpctl review

Phase 4 of the Skills + Revisions + Proposals work. Closes the reflexive
loop: Claude sessions can now propose back content (prompts or skills)
that maintainers triage via a CLI queue. The system documents itself
to Claude through the same mechanism it documents to humans.

## What's added

### propose-learnings global skill (mcpd bootstrap)
- src/mcpd/src/bootstrap/system-skills.ts — idempotent upsert, mirrors
  system-project.ts. Single skill seeded today: `propose-learnings`,
  ~430 words, explains when to engage with propose_prompt vs
  propose_skill, what makes a good proposal, what NOT to propose, and
  the review→approve flow. Priority 9, global scope.
- main.ts: `bootstrapSystemSkills(prisma)` called right after
  `bootstrapSystemProject`.

### gate-encouragement-propose system prompt
- system-project.ts gains a new gate prompt (priority 10, alongside the
  other gate-* prompts) that nudges Claude to call propose_prompt when
  it discovers a project-specific lesson. Pairs with the propose-learnings
  skill — the prompt is the trigger, the skill is the manual.

### propose_skill MCP tool (mcplocal)
- proxymodel/plugins/gate.ts: new virtual tool registered alongside
  propose_prompt. Posts to /api/v1/proposals (the new endpoint from
  PR-2) with resourceType='skill'. Tool description steers Claude
  toward propose_prompt for project-specific knowledge and reserves
  propose_skill for cross-cutting cases. propose_prompt's tool
  description is also expanded to point at the propose-learnings skill
  for guidance — the bare "creates a pending request" copy was bland
  enough that nothing in Claude's prior would actually make it engage.

### mcpctl review CLI
- New top-level command in src/cli/src/commands/review.ts.
  Subcommands:
    mcpctl review pending       List pending proposals
    mcpctl review next          Show oldest pending
    mcpctl review show <id>     Full detail
    mcpctl review approve <id>  POST /proposals/:id/approve
    mcpctl review reject <id> --reason "..."
    mcpctl review diff <id>     Side-by-side current vs proposed
- Wired into src/cli/src/index.ts. Registered after createApproveCommand
  to keep the existing project-ops `mcpctl approve promptrequest`
  command working (legacy) while the new review surface is the
  preferred path.

## Tests touched

- bootstrap-system-project.test.ts already counts via
  getSystemPromptNames() length, so it picked up the new prompt
  automatically; only the priority assertion needed nothing — the
  new prompt starts with `gate-` so the existing `gate-* → priority 10`
  invariant validates it.
- system-prompt-validation.test.ts: bumped expected length from 11→12
  and added a `toContain('gate-encouragement-propose')` assertion.

Full suite: 158 test files / 2127 tests green.

## What's NOT in this PR

- A SkillService mock-based test for the proposal approval handler —
  the PromptService approval handler is structurally identical and
  already covered; the database-backed integration is exercised in
  PR-2's tests.
- Changes to mcplocal's existing handleProposePrompt URL — it still
  POSTs to the legacy /api/v1/projects/.../promptrequests endpoint,
  which works because PR-2 left that route in place. PR-7 will
  cut mcplocal over to /api/v1/proposals along with the
  PromptRequest table rename + drop.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/mcpd/tests/system-prompt-validation.test.ts

@@ -101,10 +101,13 @@ describe('System Prompt Validation', () => {
   });
 
   describe('getSystemPromptNames', () => {
-    it('includes all 11 system prompts (5 gate + 6 LLM)', () => {
+    it('includes all 12 system prompts (6 gate + 6 LLM)', () => {
       const names = getSystemPromptNames();
       expect(names).toContain('gate-instructions');
       expect(names).toContain('gate-encouragement');
+      // PR-4: pairs with the propose-learnings global skill — sits in the
+      // gating bundle so Claude considers proposing back to mcpd.
+      expect(names).toContain('gate-encouragement-propose');
       expect(names).toContain('gate-intercept-preamble');
       expect(names).toContain('gate-session-active');
       expect(names).toContain('session-greeting');
@@ -114,7 +117,7 @@ describe('System Prompt Validation', () => {
       expect(names).toContain('llm-gate-context-selector');
       expect(names).toContain('llm-summarize');
       expect(names).toContain('llm-paginate-titles');
-      expect(names.length).toBe(11);
+      expect(names.length).toBe(12);
     });
   });