fix: verify title marker in ensure_pane() fast path (#93)

[LeoLin990405]

Summary

Fixes #93 — ensure_pane() trusts stale pane_id without title verification, routing messages to the wrong tmux pane after CCB restart.

Root cause: All 8 session modules have the same fast path that returns immediately when pane_id is alive, without checking if the title marker resolves to a different pane. When tmux recycles a pane ID to another process, messages are silently delivered to the wrong target.

Fix: Add title marker verification to the fast path in all 8 session modules. If the marker resolves to a different alive pane, update the cached pane_id and route to the correct pane.

Files changed:

• lib/gaskd_session.py (Gemini)
• lib/caskd_session.py (Codex)
• lib/oaskd_session.py (OpenCode)
• lib/daskd_session.py (Droid)
• lib/baskd_session.py (CodeBuddy)
• lib/haskd_session.py (Copilot)
• lib/laskd_session.py (Claude)
• lib/qaskd_session.py (Qwen)
• test/test_ensure_pane_stale.py (40 new tests)

Test plan

• 40 new tests: 8 session modules × 5 scenarios (marker match, stale ID switch, no marker, dead pane fallback, resolver exception)
• 175 existing tests pass (0 regressions)
• Manual: restart CCB, verify ask gemini routes to correct pane after tmux pane ID recycling