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/DATABASE.md

@@ -64,9 +64,10 @@ graph TB
 ```
 
 **Database Location:**
-- **Canonical (default):** `~/.claude/agent-dashboard/dashboard.db` — shared by `npm start`, `npm run dev`, Docker (bind mount), and the desktop app when it uses the same data dir
+- **Canonical (default):** `~/.claude/agenthub/dashboard.db` — shared by `npm start`, `npm run dev`, Docker (bind mount), and the desktop app when it uses the same data dir
 - **Override:** set `DASHBOARD_DATA_DIR` (directory) or `DASHBOARD_DB_PATH` (file path) for tests or custom deployments
 - **Legacy:** repo-local `./data/dashboard.db` is migrated into the canonical location on first launch (see `server/db.js`)
+- **Pre-rebrand:** an install that predates AgentHub keeps its data in `~/.claude/agent-dashboard/`. Server startup renames that directory to `~/.claude/agenthub/` once, before anything opens the database (`migrateLegacyDataDir()` in `server/lib/claude-home.js`). It is a same-filesystem rename, so it is instant even at multiple gigabytes and never leaves two copies. If the rename fails, or `DASHBOARD_DATA_DIR` is set, nothing moves and the old directory keeps being used.
 
 ---
 
@@ -813,13 +814,13 @@ VACUUM INTO '/backups/dashboard_20240318.db';
 ```bash
 #!/bin/bash
 # Stop application
-systemctl stop agent-dashboard
+systemctl stop agenthub
 
 # Copy database file
-cp /var/lib/agent-dashboard/dashboard.db /backups/dashboard_$(date +%Y%m%d).db
+cp /var/lib/agenthub/dashboard.db /backups/dashboard_$(date +%Y%m%d).db
 
 # Start application
-systemctl start agent-dashboard
+systemctl start agenthub
 ```
 
 ### Backup Schedule