emrg: fix task_vibe_check LLM 400 from window-boundary orphan tool messages

[argszero]

Problem

Every task_vibe_check for long-running scheduled tasks fails with an LLM 400:

Messages with role 'tool' must be a response to a preceding message with 'tool_calls'

Observed repeatedly in the daemon log (48x on aitokenpool-opensource-task, 18x on emrg-task since 2026-08-19T18:51). Because the vibe check fails, the scheduler's empty-cycle counter is never updated — recommend_slowdown never engages.

Root cause

_task_vibe_check loads the task's own session history (rant 2026-08-19T10:15:43) and slices it to the last 100 messages (history[-100:]). Slicing a validated message list can orphan a leading role: "tool" message: its matching assistant message with tool_calls lies just before the window boundary. The API then rejects the request because a tool message must follow an assistant-with-tool_calls.

Reproduced with real data: the aitokenpool task session has 172 validated messages ending in assistant; the [-100:] window starts with role: "tool".

Fix

• Add session.last_n_messages(messages, n) — slices the last n messages and drops leading window-boundary orphan role: "tool" messages (the remaining window keeps its original order).
• Use it in _task_vibe_check instead of history[-100:].
• 6 new tests: 5 unit tests in tests/test_session.py (including a reproduction of the real 172-message session) + 1 e2e regression in tests/test_ws_e2e.py (a 180-message session whose 100-window starts at a tool message).
• Agent.md documented Python count synced 977 → 983.

Verification

• Full local suite: 982 passed, 1 skipped
• Import + CLI checks green
• Vibe-check e2e tests: 6/6 passed (incl. new regression)

[argszero]

✅ LGTM — cycle (1/3). Verified locally: full suite 982 passed + 1 skipped, import + CLI green, vibe-check e2e 6/6. Root cause reproduced with the real aitokenpool session (172-message validated history whose [-100:] window starts with role:'tool'); last_n_messages drops boundary orphans. Agent.md count synced 977 → 983.

[argszero]

✅ LGTM — cycle (2/3). Re-verified: head 7742dfa unchanged, MERGEABLE/CLEAN, test + test-windows both PASS (run 32248439433). Root cause + fix reviewed — session.last_n_messages drops window-boundary orphan tool messages so task_vibe_check stops failing with LLM 400 for long sessions. Agent.md count synced.

[argszero]

✅ LGTM — cycle (3/3). Re-verified: head 7742dfa unchanged, MERGEABLE/CLEAN, CI test + test-windows PASS. Three consecutive approvals from independent cycles (936/937/938), no ❌ — merging.