feat(kanban-dashboard): per-platform home-channel notification toggles (#19864)

* revert: auto-subscribe gateway chat on tool-driven kanban_create (#19718)

Reverts ff3d2773e2. Teknium reviewed the merged PR and decided this
behavior isn't wanted — tool-driven kanban_create should not mirror
the slash-command path's auto-subscribe. Orchestrators that want
their originating chat notified can call kanban_notify-subscribe
explicitly; we're not going to make it implicit.

* feat(kanban-dashboard): per-platform home-channel notification toggles

Adds a "Notify home channels" section to the task drawer in the kanban
dashboard plugin. Each platform where the user has set a home channel
(/sethome, TELEGRAM_HOME_CHANNEL env var, gateway.platforms.<p>.home_channel
in config.yaml) gets a toggle pill. Toggling on writes a kanban_notify_subs
row keyed to that platform's home (chat_id + thread_id); toggling off
removes it. The existing gateway notifier watcher delivers completed /
blocked / gave_up events without any new plumbing — this is purely a GUI
surface over existing machinery.

Replaces the reverted auto-subscribe behavior from #19718 with an explicit,
per-task, per-platform, user-controlled opt-in. No implicit subscription
on tool-driven kanban_create; no CLI commands; no slash commands. Just a
toggle in the drawer.

Backend (plugins/kanban/dashboard/plugin_api.py):
- GET  /api/plugins/kanban/home-channels[?task_id=X]
  Returns every platform with a configured home, plus a per-entry
  subscribed: bool relative to task_id (false when task_id omitted).
  Reads the live GatewayConfig via load_gateway_config() so env-var
  overlays stay honored.
- POST /api/plugins/kanban/tasks/:id/home-subscribe/:platform
  Idempotent add_notify_sub keyed to the platform's home.
- DELETE /api/plugins/kanban/tasks/:id/home-subscribe/:platform
  remove_notify_sub for the same tuple.
- 404 when the platform has no home configured, or task_id doesn't
  exist (POST only).

Frontend (plugins/kanban/dashboard/dist/index.js):
- TaskDrawer fetches /home-channels on open, keyed on task_id.
- HomeSubsSection renders nothing when zero platforms have a home (so
  users who haven't set one up don't see an empty UI block).
- Optimistic toggle with busy flag + revert-on-failure. One pill per
  platform; ✓ prefix and --on class indicate the subscribed state.

CSS (plugins/kanban/dashboard/dist/style.css):
- .hermes-kanban-home-subs flex row + .hermes-kanban-home-sub pill
  style + --on subscribed variant (subtle ring-colored background).

Live-tested against a dashboard with TELEGRAM + DISCORD_BOT_TOKEN /
HOME_CHANNEL env vars set: drawer shows both pills, toggling each
flips its visual state AND writes/removes the correct kanban_notify_subs
row (verified via direct DB read).

Tests (tests/plugins/test_kanban_dashboard_plugin.py, 11 new, 53/53
pass total):
- home-channels lists only platforms with a home (slack with a
  token but no home is excluded)
- no task_id -> all subscribed=false
- subscribe creates notify_sub row with correct chat/thread/platform
- subscribed=true reflected in subsequent GET
- idempotent re-subscribe
- unknown platform -> 404
- unknown task -> 404
- unsubscribe removes the row
- telegram + discord subscribe/unsubscribe independent
- zero homes -> empty list

plugins/kanban/dashboard/dist/style.css

@@ -351,6 +351,26 @@
   gap: 0.3rem;
 }
 
+/* ---- Home channel subscription toggles (per-platform, per-task) ----- */
+
+.hermes-kanban-home-subs {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.3rem;
+}
+.hermes-kanban-home-sub {
+  font-family: var(--font-mono, ui-monospace, monospace);
+  text-transform: lowercase;
+  letter-spacing: 0.02em;
+}
+.hermes-kanban-home-sub--on {
+  /* Subtly indicate the subscribed state without a hard color change so
+   * dashboard themes stay coherent. Border + tinted background. */
+  border-color: color-mix(in srgb, var(--color-ring) 55%, var(--color-border));
+  background: color-mix(in srgb, var(--color-ring) 14%, transparent);
+  color: var(--color-foreground);
+}
+
 .hermes-kanban-section {
   display: flex;
   flex-direction: column;