Allow multiple ty instances via a granular per-executor lock

[bborn]

Why

Opening a second ty was refused outright ("a ty session is already running") — a blunt process-wide lock added to fix the dual-TUI interrupt-loop bug (#612). But the real constraint is narrower: only one TUI may borrow a given executor's tmux pane at a time. Two boards sitting on the kanban never actually conflict. This relaxes the lock to match the true constraint, so you can have ty open in more than one tab.

What changed

• Drop the process-wide gate (acquireTUILock + the startup refusal in cmd/task/main.go). Multiple TUIs can now run at once.

• Add a per-executor flock (acquireExecutorLock(taskID) in internal/ui): an exclusive lock on executor-<id>.lock next to the DB, taken at the single join chokepoint (joinTmuxPanes), held for the life of the detail view, released on Cleanup. When another instance owns it, the TUI refuses to join and shows:

  This executor is running in another ty instance. Close it there to view it here.

  It retries on the normal ~5s poll, so the view auto-recovers once the other instance releases the pane — no user action needed.

• Crash-safe / isolation-safe (same properties as the old lock): flock releases on process exit, and the lock lives next to the DB so a custom WORKTREE_DB_PATH (QA harness, separate project) gets its own namespace.

• Debug observability: expose pane_error / busy_elsewhere in the TUI debug state so the QA harness can assert the busy condition without screen-scraping.

The daemon, web viewer, and desktop app are unaffected — they attach non-destructively and never borrow panes.

Testing

• Unit: acquireExecutorLock — same executor refuses a second holder, different executors don't contend, re-acquire works after release (internal/ui/executor_lock_test.go).
• Real TUI on the isolated QA harness:
  1. Two TUIs coexist — both boot to the dashboard on the same DB (old lock would refuse the second).
  2. Granular refusal — with the executor lock held by another instance, opening the task shows the message and does not steal the pane (executor stays in the daemon window; the UI session has only its TUI pane).
  3. Auto-recovery — releasing the lock rejoins the executor on the next poll and clears the message.
• go vet, gofmt, and the internal/ui + cmd/task suites are green.

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