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.

skills/ship-feature-lane/SKILL.md

@@ -127,13 +127,12 @@ The e2e hook doesn't run migrations itself — it tests the already-running stac
 - `agenthub stage review --status running`, then launch a fresh-context reviewer agent (Agent tool, `subagent_type: Code Reviewer` — FOREGROUND, **never `fork`**; a reviewer who already watched you write the code is not a reviewer). Hand it ONLY: the lane's working directory, the lane spec path (requirement + acceptance criteria), the diff commands, and the brief below. Do NOT summarize your own implementation for it, and do NOT pre-empt findings — let it read the diff cold.
   - Run the **`code-review` skill at effort `high`** on the feature diff vs `origin/development` — the deterministic gate, not an ad-hoc read. No PR is open yet, so point it at the local diff: `git diff origin/development...feat/<slug>` (and `git log origin/development..feat/<slug>` for commits). ONLY if the `code-review` skill is unavailable in its session, fall back to a manual review of that diff (correctness, security, tests, migration/contract safety, this repo's `CLAUDE.md` + `.claude/rules/` boundaries).
   - Final line: `REVIEW: CLEAN` or `REVIEW: FINDINGS — <n>`, with the findings listed above it as `path:line — severity — problem — suggested fix`.
-- **Domain reviewers, in parallel with the above (same message, separate Agent calls).** Check `git diff --name-only origin/development...feat/<slug>` for touched layers and launch every matching one — FOREGROUND, fresh context, **never `fork`**, addressed as `agenthub:backend-reviewer` / `agenthub:frontend-reviewer` / `agenthub:mcp-reviewer` (plugin-qualified — no lane install needed, unlike `qc-local`/`senior-gate-reviewer`):
+- **Domain reviewers, in parallel with the above (same message, separate Agent calls).** Check `git diff --name-only origin/development...feat/<slug>` for touched layers and launch every matching one — FOREGROUND, fresh context, **never `fork`**, addressed as `agenthub:backend-reviewer` / `agenthub:frontend-reviewer` (plugin-qualified — no lane install needed, unlike `qc-local`/`senior-gate-reviewer`):
   - Any path under `server/` → `agenthub:backend-reviewer`.
   - Any path under `client/` → `agenthub:frontend-reviewer`.
-  - Any path under `mcp/` → `agenthub:mcp-reviewer`.
-  - None of those three touched (root/`skills/`/`agents/`/`scripts/`/docs-only change) → skip all three; the generic Code Reviewer above is the only gate.
-  - Hand each ONLY the lane's working directory and the diff range `git diff origin/development...feat/<slug>` — each scopes itself to its own domain's files even though the diff range is the same for all.
-  - Each ends with its own parseable line: `BACKEND-REVIEW:` / `FRONTEND-REVIEW:` / `MCP-REVIEW:` — `CLEAN` or `FINDINGS — <n>`.
+  - Neither touched (root/`skills/`/`agents/`/`scripts/`/docs-only change) → skip both; the generic Code Reviewer above is the only gate.
+  - Hand each ONLY the lane's working directory and the diff range `git diff origin/development...feat/<slug>` — each scopes itself to its own domain's files even though the diff range is the same for both.
+  - Each ends with its own parseable line: `BACKEND-REVIEW:` / `FRONTEND-REVIEW:` — `CLEAN` or `FINDINGS — <n>`.
 - Triage ALL findings — the generic review plus every domain reviewer that ran — together with **receiving-code-review** judgment (verify each, neither blind agreement nor blind rejection) and apply what's worth applying on the feature branch. Changed source → re-run **from Stage 1b**; changed only tests → **from Stage 2**. Keep the findings + your resolutions — the senior gate reads them.
 - The Stage-6 `qc-local` report covers the user-flow review for the senior gate.
 - `agenthub stage review --evidence "<total findings count across all reviewers that ran> findings, resolved"`