feat(kanban): notify_fallback — auto-subscribe channel-less task creates

[sstark21]

Summary

Tasks created from sessions with no persistent delivery channel (CLI, cron, dashboard scripts, bare API calls) silently get no auto-subscription — the gateway notifier only delivers to subscribed chats, so completion/block events from such tasks reach nobody unless someone polls the board.

This adds an opt-in kanban.notify_fallback config (format "<platform>:<chat_id>", e.g. "telegram:123456789"):

• When set, _maybe_auto_subscribe falls back to this target for channel-less creates instead of the historical no-op.
• Strictly opt-in: the empty default preserves the behaviour restored by revert: auto-subscribe gateway chat on tool-driven kanban_create (#19718) #19721 — no over-eager auto-subscription of every CLI invocation.
• A live session channel (gateway/TUI) always wins over the fallback.
• Fallback rows carry no live-session delivery metadata (thread/reply anchors) — it is a synthetic subscription.

Design rationale

The upstream revert in #19721 fixed auto-subscribing every channel-less create (over-eager, #19718). The gap that remains is intentional for defaults but leaves self-hosted single-operator boards (e.g. one human, one Telegram bot, tasks created via hermes kanban create from SSH) with zero notification coverage. A config-gated fallback keeps the default conservative while giving single-operator deployments a one-line fix instead of a per-task notify-subscribe habit or an external watchdog script.

Config example

kanban:
  notify_fallback: "telegram:388101199"

Test plan

• test_channelless_create_noop_without_fallback — default unchanged (no fallback → no sub)
• test_channelless_create_uses_notify_fallback — fallback configured → sub written with platform/chat from config, no delivery metadata
• test_notify_fallback_ignored_when_session_has_channel — live session channel wins
• Full tests/tools/test_kanban_tools.py (32 tests) + tests/hermes_cli/test_config.py green via scripts/run_tests.sh
• Docs table in website/docs/user-guide/features/kanban.md updated