mcp: disk-cache tool schemas + connect-on-first-call (#206 Phase 2)

Infrastructure for advertising MCP tools at spawn without blocking on
connection, so the agent is reachable immediately AND there is no
prompt-cache miss (the previous fix accepted one intentional miss).

- schema_cache: new `~/.jcode/mcp-schema-cache.json`. Caches each
  server's `tools/list` schemas keyed by a fingerprint of its launch
  config (command + args + sorted env + shared). A config change changes
  the fingerprint and invalidates the stale entry, so we never advertise
  tools for a reconfigured server. The cache is a hint; live connections
  reconcile it. Full unit tests (fingerprint stability/sensitivity, env
  order-independence, fingerprint-gated reads, change detection, pruning,
  load/save round-trip).
- protocol: derive `Serialize` on `McpToolDef` so schemas can be cached.
- manager: `call_tool` now connects-on-first-call — if a server is
  configured but not yet connected (because we advertised cached tools
  early), it connects first, bounded by a 30s timeout, then retries.
  Clean, server-named tool errors on connect failure/timeout; never
  hangs startup. Added `ensure_server_connected(timeout)`. Tests cover
  unconfigured-server errors and bounded failure for a broken server.
- tool: `create_mcp_tools_from_cached()` builds proxy tools from cached
  schemas without a live connection (they connect on first call).

This is the reusable layer; the registration wiring lands next.