refactor: finish the AgentHub rename and drop the MCP server

Two leftovers from the rebrand, done together because they touch the same
docs and manifests.

**Data directory.** `agent-dashboard` could not simply be renamed in the
source: it names a real directory holding the live SQLite database. Startup
now moves it once, via `migrateLegacyDataDir()` in `server/lib/claude-home.js`
— a same-filesystem rename, so it is instant even at multiple gigabytes and
cannot leave two divergent copies. `getDataDir()` stays PURE and resolves to
the legacy path until the move happens; the first version of this put the
rename inside the getter, which meant `npm run test:server` relocated the
developer's real database as a side effect. The call sits behind
`require.main === module` and ahead of the router requires, because those pull
in db.js, which opens SQLite at module load. A failed rename is silent and the
old directory keeps being used: an unfashionable directory name beats a blank
database beside an unreachable one. Same treatment for the port-discovery file
(`.agent-dashboard.json` → `.agenthub.json`, old one dropped on write).

Not renamed on purpose: the Sagi localStorage keys (renaming silently resets
everyone's prefs) and the PWA manifest `id` (changing it orphans an installed
app).

**MCP server.** 4 tool calls in 46 days across 577,132 events, against 4,556
lines of TypeScript, a 2 MB committed build artifact, and a pre-commit guard
that had already blocked commits twice. Deleted: `mcp/`, the maintainer skills
and `mcp-reviewer` agent, the build check, `plugins/agenthub/.mcp.json`,
`docs/MCP.md`, 12 npm scripts, 5 Makefile targets, and every doc section
describing it.

Kept, because they are unrelated things sharing the name: the Config
Explorer's MCP tab and its API, per-lane Playwright MCP, the agenthub-config
plugin's MCP audit, and the historical `mcp__plugin_agenthub_*` events that
analytics must keep attributing.

Verified: server 1432/1432, client 459/459, tsc clean, prettier clean. Both
directions of the migration proven — a sandbox-HOME test run leaves the legacy
directory untouched, and a real boot moves it with its contents intact.

.devcontainer/devcontainer.json

@@ -5,7 +5,7 @@
   // open the repo in a GitHub Codespace. It changes nothing for host-based dev.
   //
   // Covers the full project: the Express server (4820), the React/Vite client
-  // (5173), the MCP server, and the VS Code extension. The native `better-sqlite3`
+  // (5173) and the VS Code extension. The native `better-sqlite3`
   // addon builds here (build-essential + python3 are installed in the Dockerfile).
   //
   // NOTE (issue #193): Claude Code hooks are a HOST-side concern. This container
@@ -40,7 +40,7 @@
     }
   },
 
-  // Install root + client + vscode-extension deps and build the MCP server.
+  // Install root + client + vscode-extension deps.
   // Never installs Claude Code hooks (host-only — see note above).
   "postCreateCommand": "bash .devcontainer/post-create.sh",
   "waitFor": "postCreateCommand",