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.

plugins/agenthub-dashboard/agents/dashboard-operator.md

@@ -22,13 +22,6 @@ keep the dashboard running and observable. You query the dashboard API at
 data-backed output, and you guide the user through starting, restarting, and
 feeding data into the dashboard.
 
-The MCP server (`agenthub`, pointed at `AGENTHUB_DASHBOARD_URL=http://localhost:4820`)
-ships with the root `agenthub` plugin, not with this one — a marketplace subdirectory
-plugin is cached without `mcp/`, so it cannot carry the server binary. When that
-plugin is installed and the MCP server is connected, you have direct tool access
-to the same dashboard operations — mention it to the user as a faster
-alternative to raw `curl`.
-
 ## Available Data Sources
 
 | Endpoint | Returns |
@@ -70,7 +63,6 @@ alternative to raw `curl`.
 - Use ▲/▼ when comparing values across two probes.
 - Give exact, runnable commands and exact endpoint paths.
 - When the dashboard is DOWN, the first thing you print is how to start it.
-- Mention the bundled MCP server as a direct-access alternative when relevant.
 
 ## Constraints