fix(kanban): add heartbeat grace window to prevent reclaiming slow workers (#23025)

[KhanCold]

Bug Description

Kanban workers on the default board repeatedly entered a stall loop:

1. Worker spawns (status: running)
2. Worker produces no output
3. Worker is reclaimed after ~15 min (status: reclaimed, error: stale_lock)
4. Dispatcher immediately respawns a new worker
5. New worker immediately hits the same pattern

This was particularly bad for slow models like kimi-k2.6.

Root Cause

release_stale_claims() reclaimed any running task whose claim_expires had passed, regardless of whether the worker had heartbeated recently. The default claim TTL is 15 minutes, but slow models can take longer between heartbeats.

Fix

Add a 5-minute heartbeat grace window to release_stale_claims():

• If a worker heartbeated within the last 5 minutes, skip reclaim even if claim_expires has passed
• Workers that are truly stuck (no heartbeat for >5 min) are still reclaimed
• This gives slow-model workers extra time while still catching dead workers

Tests

Added tests/hermes_cli/test_kanban_stale_claim_grace_regression_23025.py with 3 tests:

• Does NOT reclaim when worker heartbeated 2 minutes ago
• DOES reclaim when worker last heartbeated 10 minutes ago
• DOES reclaim when worker never heartbeated

All tests pass.

Fixes #23025

[alt-glitch]

Duplicate of PR #23071 which addresses the same issue #23025 with a PID-liveness approach rather than heartbeat grace window.