mcp: collapse app + launcher to worker-only

The idalib worker is now the sole backend for opening a binary, so remove the
ida-pro-mcp code paths from the hot path:

* app.py: IdaTui.__init__ drops url/db/ensure_server/backend (now just
  open_path/keepalive/rpc_path/ttl); _connect calls _open_worker_client directly;
  _open_mcp_client deleted; _reconnect respawns the worker only; self.client and
  _after_reconnect typed WorkerClient. No more IDAClient import.
* launch.py: rewritten worker-only -- validate the binary, sweep stale locks,
  spawn the TUI (which starts the private worker behind its overlay). The whole
  supervisor dance (_ensure_server/_start_supervisor/_open_binary/
  _existing_session) is gone; `ida-tui foo.elf` is the one usage.
* tests/test_scenarios.py: pilot is worker-only (run(binary); binary via
  positional/--worker/--binary, defaults to targets/echo).

Verified: app + launch + pilot import and construct; pilot lists 31 scenarios.
The mcp modules (client.py/pane.py/tui.py/spawn.sh) still exist as dead code and
are deleted in the next commit. Worker pilot (134/2-known-flakes) still the gate.

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