fix: notify gateway-origin kanban_create tasks

[finnnagen]

Summary

• Auto-subscribe structured kanban_create tasks to native Kanban completion notifications when a concrete messaging gateway origin is present.
• Keep CLI/local/no-origin kanban_create calls board-only and return explicit notify_subscribed / notify_error fields.
• Add regression coverage for gateway-origin subscription creation and no-origin behavior.

Diff summary

• tools/kanban_tools.py: adds _gateway_notify_origin() using gateway.session_context.get_session_env(...); after kb.create_task(...), calls kb.add_notify_sub(...) only when platform/chat origin exists; preserves successful task creation if subscription fails and reports notify_error; documents gateway-origin behavior in the structured tool schema.
• tests/tools/test_kanban_tools.py: asserts default create results are board-only, tests gateway-origin native kanban_notify_subs row creation, and tests no-origin calls do not create subscriptions.

Safety / privacy check

• Uses existing Hermes Kanban primitives only: kb.create_task(...), kb.add_notify_sub(...), kanban_notify_subs, and gateway session contextvars.
• No wrapper services, custom callback protocols, stdout scraping, or setup-specific routing.
• Diff privacy scan checked for user-specific names, business/project-specific names, local filesystem paths, host/workspace identifiers, real-looking Discord snowflakes, phone-number patterns, and raw token/secret assignments. All returned 0 hits.
• Test IDs are synthetic placeholders (chat-123, thread-456, user-789).

Test plan

• python -m pytest tests/tools/test_kanban_tools.py::test_create_happy_path tests/tools/test_kanban_tools.py::test_create_auto_subscribes_gateway_origin tests/tools/test_kanban_tools.py::test_create_stays_board_only_without_gateway_origin -q -o 'addopts=' — passed.
• python -m ruff check tools/kanban_tools.py tests/tools/test_kanban_tools.py — passed.
• Temp-DB smoke test: gateway-origin structured create produced one kanban_notify_subs row; no-origin structured create produced none.

Note

• Full tests/tools/test_kanban_tools.py currently has one unrelated existing failure in test_heartbeat_extends_claim_expires on this checkout; the focused create/notification tests pass.

[alt-glitch]

Likely duplicate — this re-implements the kanban_create auto-subscribe feature from #19718 (merged then reverted by #19721). Multiple competing open PRs exist for the same feature: #27064, #25195, #24307, #21523, #22190. Please coordinate with those authors or close in favor of one.