mcp: advertise cached tools at spawn + reconcile cache on connect (#206 Phase 2)

Wire the schema cache into session MCP registration so MCP tools are
visible from the very first turn with zero provider prompt-cache miss in
the common (warm-cache) case.

- Advertise-early: before spawning the background connect task,
  `register_mcp_tools` loads the disk schema cache and synchronously
  registers proxy tools for every configured server that has cached
  schemas matching its current config. These land in the first locked
  tool snapshot, so the model sees MCP tools immediately and the
  connect-on-first-call proxy handles actual invocation. An McpStatus
  event reflects the advertised counts right away.
- Reconcile: after the background connection completes, the live schemas
  are written back to the cache (per-server, under the current
  fingerprint) and servers no longer configured are pruned, so the next
  spawn advertises an up-to-date set. Live registration re-registers
  proxies idempotently, refreshing any drifted schema.
- Cold start / reconfigured servers (no matching cache entry) fall back
  to the existing post-connect registration plus the one-shot
  late-register rebuild, which now fires at most once.