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.

scripts/plugin-open.js

@@ -4,7 +4,7 @@
  * @description Builds the dashboard UI bundle for a plugin install and prints
  * the dashboard URL. `scripts/plugin-bootstrap.js` calls `buildClient` eagerly
  * on session start, so client-only routes (e.g. `/run`) work the moment
- * `claude` is started, not just the API and MCP tools. `/agenthub-open` re-runs
+ * `claude` is started, not just the API. `/agenthub-open` re-runs
  * this standalone — a no-op when the bundle already matches — as a fallback
  * for `--force` rebuilds or a first build that failed during bootstrap, and it
  * re-runs bootstrap first when the live server belongs to a retired plugin root
@@ -92,7 +92,7 @@ if (require.main === module) {
 
   const result = buildClient({ force: process.argv.includes("--force") });
   if (result === "failed") {
-    console.error("Client build failed — the API and MCP tools still work.");
+    console.error("Client build failed — the API still works.");
     process.exitCode = 1;
   } else {
     if (result === "built") console.log("Dashboard UI built.");