fix(kanban): align health and diagnostics with guards

[KostaGorod]

Summary

• Make spawnability health use the same ready/review guard predicate as dispatch, preserving review-lane semantics.
• Replace misleading stranded-ready reassign advice with the recorded guard reason and timestamp when a guard event has no later superseding event. This is historical evidence, not proof of an active guard; reads remain non-mutating.
• Fixes Kanban health and stranded-card diagnostics disagree with lane-specific respawn guards #126263.

The overlapping work is complementary: #44899 covers older ready-only spawnability behavior; #65996 surfaces active-PR guard output; #123964 proposes broader board health/escalation. This PR is limited to the health probe and per-task stranded diagnostic. It does not change guard policy or create escalation cards.

Test plan

Focused verification

• HERMES_PYTHON=/home/ai/.hermes/hermes-agent/venv/bin/python scripts/run_tests.sh tests/hermes_cli/test_kanban_diagnostics.py tests/hermes_cli/test_kanban_dispatch_claim_allowlist.py tests/hermes_cli/test_kanban_db.py tests/hermes_cli/test_kanban_cli_dispatch_passthrough.py — 70 passed, 0 failed, 1 skipped (Windows-only host case).
• git diff --check — passed.

Remaining: reason-specific guard clearing, guard-window expiry, same-second event ordering, and fresh-ready-age reset coverage are not yet included in this focused draft. CI and independent review are pending.

[KostaGorod]

Overlap note: #44899 and #65996 address related but distinct guard behavior and active-PR observability. This draft is limited to the residual lane-aware health probe and task diagnostic; current focused tests pass, while reason-specific clearing/expiry/order and fresh-ready-age coverage remain unfinished as stated in the body.