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.

bin/agenthub.js

@@ -28,7 +28,7 @@
  *
  * Port resolution mirrors the hook handler: an explicit
  * CLAUDE_DASHBOARD_PORT / DASHBOARD_PORT env var wins, otherwise the live
- * server is discovered from ~/.claude/.agent-dashboard.json (written by every
+ * server is discovered from ~/.claude/.agenthub.json (written by every
  * running dashboard, PID-liveness-checked on read), falling back to 4820.
  *
  * Read commands are always safe. Mutating commands are explicit user actions
@@ -1370,7 +1370,7 @@ async function cmdReinstallHooks() {
  * the live URL when a server is already up. The child is fully detached with
  * its output appended to data/agenthub-server.log, so closing this terminal does
  * not stop the dashboard; stop it later with `kill <pid>` (the PID is
- * printed and registered in ~/.claude/.agent-dashboard.json).
+ * printed and registered in ~/.claude/.agenthub.json).
  */
 async function cmdStart(flags) {
   if (await serverIsUp()) {
@@ -3025,7 +3025,7 @@ function catalogRow(name, args, desc) {
 function helpFooter() {
   return `
 ${c.bold("Server discovery:")} CLAUDE_DASHBOARD_PORT / DASHBOARD_PORT env vars win,
-otherwise the live server is found via ~/.claude/.agent-dashboard.json,
+otherwise the live server is found via ~/.claude/.agenthub.json,
 falling back to http://127.0.0.1:4820.
 
 ${c.bold("Output:")} colors auto-enable on a TTY and turn off when piped.