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-bootstrap.js

@@ -13,7 +13,7 @@
  * reboots without Claude Code being opened first; otherwise a detached spawn
  * scoped to this session, same as before.
  *
- * Everything writable lives under `~/.claude/agent-dashboard/runtime/` — NOT in
+ * Everything writable lives under `~/.claude/agenthub/runtime/` — NOT in
  * the plugin cache, which Claude Code garbage-collects and replaces wholesale
  * on every plugin update.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
@@ -416,7 +416,7 @@ function log(rt, line) {
  * clone on every session. `/agenthub-update` (`--force`) retries.
  *
  * Best-effort throughout. Nothing here may throw or block: the dashboard,
- * hooks, CLI and MCP all work without these plugins.
+ * hooks and CLI all work without these plugins.
  *
  * @returns {"off"|"nothing-missing"|"installed"|"failed"|"error"} or the previously
  *   recorded outcome when this run had nothing to settle.
@@ -499,10 +499,7 @@ function bootstrap(rt = runtimeDir(), { force = false } = {}) {
       logPath: path.join(rt, "client-build.log"),
     });
     if (uiResult === "failed") {
-      log(
-        rt,
-        "dashboard UI build failed — API and MCP tools still work; run /agenthub-open to retry"
-      );
+      log(rt, "dashboard UI build failed — the API still works; run /agenthub-open to retry");
     } else if (uiResult === "built") {
       log(rt, "built the dashboard UI bundle");
     }