Keep Stop/idle partials, cancel tools honestly, unblock CI

[wu1w]

Personal-desktop smoothness: Stop/idle/delete feel like a mainstream agent, secrets are stripped silently, and CI lint gates are unblocked. No new approval gates, confirm dialogs, or capability walls.

What changed

Stop / idle / session delete

• sync_response idle now keeps the streamed partial (same path as status: idle / Stop fallback) instead of wiping it before history lands.
• In-flight tools are marked cancelled, not completed-then-deleted. The cancelled rows ride on the kept partial bubble.
• Backend Stop no longer commits prefetched readonly results. Serial tool calls are cancelled when _should_stop flips (process trees already die on CancelledError).
• Composer send is not blocked while Stop is in flight, so the input does not sit idle-spinning.
• Successful session delete immediately drops tevarn-chat-draft:<id> and the stream-cache entry. No extra confirm or delay.

Silent secret strip

• Local python / command spawn inherits host env minus control-plane keys (TEVARN_JWT_SECRET, desktop HMAC, admin password, encryption/RPC secrets). PATH, HOME, and user-intended env (e.g. OPENAI_API_KEY) stay. No permission prompt.

CI without policy churn

• useColResize syncs the width ref in an effect (React-legal). Did not downgrade react-hooks/refs.
• Removed unused f-string prefixes in backend/agent/loop.py (Ruff F541). Pytest can run again after Ruff.
• Typecheck follow-up: chat page no longer shadows useT() with a setTimeout id; CronWebhookPanel imports useT so existing t() calls typecheck.

Hang-avoidance

• Image aiohttp sessions use ClientTimeout(total=120, connect=10). No retries.
• emit_run_event spill I/O and file_write / file_edit disk I/O go through asyncio.to_thread.

What was verified locally

• ruff check on the F541 file and all touched Python files: clean.
• ESLint on the touched frontend files: 0 errors.
• tsc --noEmit in frontend/: clean after the typecheck follow-up.
• Pytest (41 tests): test_loop_stop_ux, test_tool_parallel (including new stop/cancel cases), test_tool_spawn_env, test_hang_avoidance — all green.

Deliberately skipped

• Rust kernel lock / court (other agent).
• JobBackend filesystem isolation, extra_roots confirms, MCP allowlists, steward/_workforce stripping, multi-user IDOR checks — those add walls or hot-path gates.
• Mermaid XSS / openExternal / assertTrustedIpc — not one-line-safe for current UX.
• Rewriting ~50 Chinese snapshot / budget / tool-policy tests — out of scope; this PR only unblocks Ruff so those can surface again.
• New confirms, capability gates, dependency adds, version bumps.
• channels/page.tsx no-explicit-any warnings — not a typecheck error.

How to review

Focus on frontend/app/chat/page.tsx, backend/agent/phases/tool_round.py, backend/core/host_commands.py, and the two CI one-liners. The product goal is fail-open desktop smoothness: Stop keeps what you already saw, delete is instant, tools that need PATH still work.