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/LANES.md

@@ -1370,7 +1370,7 @@ The lane card shows a lock badge when the lane **holds** a named lock (its `lane
 
 The **ship-feature-lane** skill is the ported Shipyard driving skill for autonomous end-to-end feature pipelines in a single AgentHub lane. It coordinates the entire development flow: implementation (TDD) → independent unit tests → pre-push CI gates + dev preflight → e2e testing → code review → local QC → senior gate authorization → PR push and publication → CI watch → final report.
 
-**Every stage whose job is to judge the work runs in a fresh-context subagent, never in the driving session.** The plan debate (Stage 0b), unit tests (Stage 1b), the e2e run and its triage (Stage 3), code review (Stage 4), browser QC (Stage 6), the senior gate (Stage 7), and a re-triage of CI that stayed red after a fix (Stage 10) are each dispatched with the Agent tool to a subagent that starts with an empty context and receives only artifacts it can verify itself — working directory, lane spec, diff range, commands, and the other agents' reports verbatim, never the driving session's paraphrase of why a finding was fine. The stages the driving session keeps are the ones that *do* rather than judge: intake, implementation, the mechanical gates, publishing, the CI/PR watch, and applying every fix the judges ask for. `subagent_type: fork` is explicitly forbidden for these: a fork inherits the implementing session's context, so it inherits the belief that the code is already correct. The driving session implements and applies fixes; it never grades its own work. Stages 1b/3 use the generic `general-purpose` type (nothing to install). Stage 4 uses the generic `Code Reviewer` type as its catch-all, plus — in parallel, routed by which of `server/`, `client/`, `mcp/` the diff touches — the plugin-qualified `agenthub:backend-reviewer` / `agenthub:frontend-reviewer` / `agenthub:mcp-reviewer` domain reviewers (nothing to install; `agenthub:` addressing works from any lane). Stages 6/7 use the lane-installed `qc-local`/`senior-gate-reviewer` agents.
+**Every stage whose job is to judge the work runs in a fresh-context subagent, never in the driving session.** The plan debate (Stage 0b), unit tests (Stage 1b), the e2e run and its triage (Stage 3), code review (Stage 4), browser QC (Stage 6), the senior gate (Stage 7), and a re-triage of CI that stayed red after a fix (Stage 10) are each dispatched with the Agent tool to a subagent that starts with an empty context and receives only artifacts it can verify itself — working directory, lane spec, diff range, commands, and the other agents' reports verbatim, never the driving session's paraphrase of why a finding was fine. The stages the driving session keeps are the ones that *do* rather than judge: intake, implementation, the mechanical gates, publishing, the CI/PR watch, and applying every fix the judges ask for. `subagent_type: fork` is explicitly forbidden for these: a fork inherits the implementing session's context, so it inherits the belief that the code is already correct. The driving session implements and applies fixes; it never grades its own work. Stages 1b/3 use the generic `general-purpose` type (nothing to install). Stage 4 uses the generic `Code Reviewer` type as its catch-all, plus — in parallel, routed by which of `server/`, `client/` the diff touches — the plugin-qualified `agenthub:backend-reviewer` / `agenthub:frontend-reviewer` domain reviewers (nothing to install; `agenthub:` addressing works from any lane). Stages 6/7 use the lane-installed `qc-local`/`senior-gate-reviewer` agents.
 
 ### Getting the skill (nothing to do, if you run the plugin)