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.

docs/HOOKS.md

@@ -573,13 +573,13 @@ function sendToServer(hookType, payload) {
 > 1. If `CLAUDE_DASHBOARD_PORT` is set in the environment, the handler treats
 >    it as an explicit operator override and POSTs to that single port —
 >    no discovery, no fan-out (useful for tests and container setups).
-> 2. Otherwise it reads `~/.claude/.agent-dashboard.json`, a JSON document
+> 2. Otherwise it reads `~/.claude/.agenthub.json`, a JSON document
 >    that lists every dashboard server currently running on the machine.
 >    Each server appends its `{port, pid, startedAt, dataDir}` entry on
 >    startup and removes it on a clean shutdown. The handler **prunes any
 >    entry whose PID is no longer alive** and POSTs the hook payload to one
 >    port per **unique SQLite data directory** (lowest port wins when Docker
->    and `npm run dev` share `~/.claude/agent-dashboard`).
+>    and `npm run dev` share `~/.claude/agenthub`).
 > 3. If neither yields a target, the handler falls back to `4820`.
 >
 > Dashboards with **different** databases (e.g. the packaged desktop app using