merge: AgentProject membership — the junction is now load-bearing

Merges feat/agent-project-membership (task 166/152). An agent can finally join
many projects: membership routes, per-membership role and optional brain
override, chat resolving context from the membership, and CLI attach-agent /
detach-agent / get memberships. The legacy Agent.projectId fallback is kept for
this release as the schema comment promises.

Conflict resolved in main.ts: that branch added its RBAC case to
mapUrlToPermission inline, while this branch had extracted the function to
middleware/url-permission.ts so it could be tested. Kept the extraction and
ported the case into it — joining a team is `edit:agents:<name>`, not
`create:agents`, which is the authority to mint a whole agent.

Also fixes something the extraction makes possible. security.test.ts contained a
hand-copied reproduction of mapUrlToPermission, because main.ts self-invokes
main() and nothing in it was importable. A security test asserting against a
copy stops testing the system the moment the real one changes, and this copy had
already drifted. It now imports the real function; all 18 assertions still pass,
against the code that actually runs.

Local: build clean, workspace 2550 passed, lint unchanged at 794 errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4wNHWf7xSwnZCWpJcyv9p

.taskmaster/tasks/task_057.md

@@ -0,0 +1,50 @@
+# Task ID: 57
+
+**Title:** Add linkStatus to prompt GET responses
+
+**Status:** done
+
+**Dependencies:** 56 ✓
+
+**Priority:** medium
+
+**Description:** Modify the GET /api/v1/prompts endpoint to include linkStatus (alive/dead/unknown) for linked prompts by checking link health.
+
+**Details:**
+
+1. Update `/src/mcpd/src/routes/prompts.ts` GET endpoint:
+   ```typescript
+   fastify.get('/api/v1/prompts', async (request, reply) => {
+     const prompts = await promptService.findAll(filter);
+     
+     // Check link status for linked prompts
+     const promptsWithStatus = await Promise.all(
+       prompts.map(async (p) => {
+         if (!p.linkTarget) {
+           return { ...p, linkStatus: null };
+         }
+         const status = await linkResolver.checkLinkHealth(p.linkTarget);
+         return { ...p, linkStatus: status };
+       })
+     );
+     
+     return reply.send(promptsWithStatus);
+   });
+   ```
+
+2. Consider caching link health to avoid repeated checks
+
+3. Add `linkStatus` field to prompt response schema:
+   - `null` for non-linked prompts
+   - `'alive'` for working links
+   - `'dead'` for broken links
+   - `'unknown'` for unchecked links
+
+**Test Strategy:**
+
+- Unit test: Non-linked prompt has linkStatus=null
+- Unit test: Linked prompt with working link has linkStatus='alive'
+- Unit test: Linked prompt with broken link has linkStatus='dead'
+- Unit test: Link health cached to avoid repeated checks
+- Unit test: All prompts in response have linkStatus field
+- Integration test: End-to-end GET with linked prompts