worker: surface the real startup failure (not just "code 1")

The worker's stderr was swallowed by the TUI, so an open failure showed only
"worker exited during startup (code 1)". Now:

* WorkerClient captures the worker's stdout+stderr to /tmp/idatui-worker-*.log
  and, on a startup exit, surfaces the last meaningful line in the error (the
  worker prints a clean 'WORKER-FATAL: ...' marker; _log_tail prefers it).
* worker.py wraps main() to print that marker + traceback before exiting 1, and
  gives an ACTIONABLE open error: "failed to open <bin>: the .i64 is likely held
  by a running ida-mcp worker (pkill -f idalib) or wedged (delete .id0/.id1/
  .id2/.nam/.til)". Also calls ida_auto.auto_wait() after open to fully match
  ida-mcp's session manager (open_database + auto_wait).

Root cause of the reported failure is almost certainly a leftover ida-mcp worker
still holding bash's .i64 from earlier --backend mcp runs: idalib can't open a
database another process has locked. Fix: pkill -f idalib, then retry
--backend worker; the error message now says so instead of "code 1".

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