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_055.md

@@ -0,0 +1,61 @@
+# Task ID: 55
+
+**Title:** Implement system prompt fetching and caching in mcplocal
+
+**Status:** done
+
+**Dependencies:** 54 ✓
+
+**Priority:** high
+
+**Description:** Add functionality to mcplocal router to fetch system prompts from mcpd and cache them with 60s TTL, supporting template variable replacement.
+
+**Details:**
+
+1. Add system prompt fetching in `/src/mcplocal/src/router.ts`:
+   ```typescript
+   private systemPromptCache: Map<string, { content: string; expiresAt: number }> = new Map();
+   
+   async getSystemPrompt(name: string): Promise<string> {
+     const cached = this.systemPromptCache.get(name);
+     if (cached && cached.expiresAt > Date.now()) {
+       return cached.content;
+     }
+     
+     const prompts = await this.mcpdClient.fetch(
+       '/api/v1/projects/mcpctl-system/prompts/visible'
+     );
+     const prompt = prompts.find(p => p.name === name);
+     if (!prompt) {
+       throw new Error(`System prompt not found: ${name}`);
+     }
+     
+     this.systemPromptCache.set(name, {
+       content: prompt.content,
+       expiresAt: Date.now() + 60000,  // 60s TTL
+     });
+     
+     return prompt.content;
+   }
+   ```
+
+2. Add template variable replacement:
+   ```typescript
+   replaceTemplateVariables(content: string, vars: Record<string, string>): string {
+     return content
+       .replace(/\{\{prompt_index\}\}/g, vars.prompt_index || '')
+       .replace(/\{\{project_name\}\}/g, vars.project_name || '')
+       .replace(/\{\{matched_prompts\}\}/g, vars.matched_prompts || '')
+       .replace(/\{\{remaining_prompts\}\}/g, vars.remaining_prompts || '');
+   }
+   ```
+
+**Test Strategy:**
+
+- Unit test: System prompt fetched from mcpd
+- Unit test: Cached prompt returned within TTL
+- Unit test: Cache miss triggers fresh fetch
+- Unit test: Missing system prompt throws error
+- Unit test: Template variables replaced correctly
+- Unit test: Unknown template variables left as-is
+- Integration test: End-to-end fetch and cache