feat(cmd): mount the read plane, MCP and GraphQL surfaces

The three Phase 2 surfaces were built but never served: each was written
under an instruction not to touch cmd/, so every one reported its mounting
call and none of them wired it. The daemon answered /healthz and 404'd
everything else.

They share one search.Index, because bleve is single-writer and a second
Open on the same directory is wrong rather than merely wasteful.

Route order is load-bearing: /mcp and /query register before the web UI
mounts at /, which would otherwise swallow them as document paths — the
router has no reason to think 'mcp' is not a space name.

The MCP handler takes the configured origin as its Host allowlist, which
is why Traefik must pass the Host header through.

feat: hooks — the receive path, its daemon RPC, and the specsrht daemon

The hooks are thin shims that RPC into the running daemon over a unix
socket under the repos root, so validation lives in one place and the
push path cannot drift from the API. They fail closed: an unreachable
daemon rejects the push.

Three hooks are installed, not two, because of two properties of git
verified against 2.55 rather than inferred:

  - GIT_PUSH_OPTION_* reaches pre-receive and post-receive only; the
    update hook observably runs without them, so it cannot see
    --push-option=skip-validation on its own.
  - during pre-receive the pushed objects are still in receive-pack's
    quarantine and unreadable by any other process, so the daemon
    cannot validate there. git migrates them out before the first
    update hook, which makes update the earliest hook that works.

pre-receive therefore forwards the push options and the ref list; the
daemon records them keyed by (repository, receive-pack pid) and by the
exact ref update; update does the rejecting; post-receive notifies.
An update with no recorded pre-receive phase is refused rather than
assumed unskippable.

Each hook is a symlink to the specsrht binary, which dispatches on the
name git invoked it as. Install also sets receive.advertisePushOptions,
without which the escape hatch fails client-side. The daemon refreshes
every space's hooks at startup and treats a failure as fatal.

cmd/specsrht validates every config key at once before opening
anything, serves /healthz on -b (default localhost:5091), runs the
reconciler, and warm-shuts-down on SIGINT and SIGTERM alike by
bridging SIGTERM into the SIGINT core-go's server.Run waits for.

Tested with a real git push against a real bare repo with the hooks
installed: a valid push lands, bad frontmatter is rejected with a
readable message, skip-validation waives it, a force-push to the
approved branch is refused with or without skip-validation, a mistyped
push option is refused rather than ignored, and a push with no daemon
is rejected.