fix: attribute /clear transcript to the agent whose activity stopped, not the active terminal

[yshyuk]

Fixes #251

Problem

With two Claude Code sessions in the same VS Code window (heuristic mode), running /clear in one session can swap the two characters' terminal bindings: clicking the busy-looking character focuses the cleared terminal and vice versa. See #251 for a detailed report and reproduction.

Cause

The heuristic /clear detection in server/src/fileWatcher.ts gated the claim of a freshly created /clear transcript on activeAgentIdRef.current === agentId — i.e. whichever agent owns the active terminal claims the new file. If /clear runs in a background session while the user is focused on a busy one, the busy agent claims the cleared session's transcript as soon as it goes idle for CLEAR_IDLE_THRESHOLD_MS (2 s, e.g. between tools), rebinding itself to the other session. Both agent records end up with correct-looking statuses but crossed terminalRefs — exactly the swap described in the issue.

A side effect of the old gate: a /clear in a non-active terminal was never detected at all.

Fix

Replace the active-terminal gate with pickClearClaimant(): among the eligible agents (internal, hookless, idle past the threshold), the one whose lastDataAt is closest to the new file's creation time (birthtimeMs) claims it. The session that ran /clear stopped writing its old transcript at exactly the moment the new file was created, so file-creation-time proximity is a far more reliable attribution signal than terminal focus. Each agent's own poll makes the decision, so no new coordination is needed; a poll simply skips the file when another agent is the better match.

The hooks path is unaffected — it already attributes /clear correctly via the per-agent pendingClear flag on SessionEnd(reason=clear).

Tests

• New unit suite server/__tests__/pickClearClaimant.test.ts (5 cases): single-agent claim, best-match wins over the polling agent, polling agent keeps the file when it is the best match, busy agents excluded, external/hook-driven/terminal-less agents excluded.
• npm run test:server — 283 passing.
• npm run compile (types + lint + build) clean.