fix(agent): bind kanban worker guidance to dispatched tasks

[jayleaton]

Summary

• gate dispatcher-only Kanban lifecycle guidance on a nonempty task binding and canonical dispatcher ownership
• keep configured orchestrators able to inspect explicit task IDs without telling them to orient a nonexistent current task
• cover unbound cron final responses, real workers, inherited worker context, and explicit task-id inspection

Verification

• scripts/run_tests.sh tests/agent/test_system_prompt.py tests/tools/test_kanban_tools.py tests/cron/test_cron_kanban_env_isolation.py -q (106 passed)
• ruff check on all changed Python files
• python -m compileall -q on all changed Python files
• independent review: PASS, no blocking security or logic findings

Rollout

No gateway process was replaced or restarted. Activation remains deferred while the existing cumulative gateway verification window is active.

[alt-glitch]

This was generated by AI during triage.

Related: #64186 (earlier fix, same files) and #95857 both gate KANBAN_GUIDANCE for the same problem. This PR adds a stricter gate (task binding + dispatcher-ownership check, fail-closed) and drops the tool-presence fallback in system_prompt.py, so it is broader than #64186 rather than a duplicate. Reviewers may want to pick one of the three.