feat(sidebar): add show_kanban_sessions toggle to hide kanban worker sessions (#6780)

* feat(sidebar): add show_kanban_sessions toggle to hide kanban worker sessions

* fix(sidebar): kanban source-filter override + filtered-view cap (cron/webhook parity)

Gate re-review (Codex) found the kanban-hide addition missed cron/webhook
parity for the source-filter case:
- An explicit kanban source_filter now overrides the default hide (threaded
  source_filter into _dedupe_cli_sidebar_sessions_for_api, normalized with
  strip().lower() to match the loader), so a deliberate kanban view shows
  those rows even with the toggle off. Scoped to the matching source only.
- A dedicated KANBAN_PROJECT_CHIP_LIMIT bounds a filtered kanban-only view,
  mirroring CRON/WEBHOOK_PROJECT_CHIP_LIMIT.

Deliberately NOT changing the general exclude_sources: the original PR lets
kanban load on the source_filter=None path so the toggle-ON case works (the
hide is applied by _hide_from_default_sidebar only when the toggle is off).
Excluding kanban there would save the 20-row budget but break toggle-on
unless a bounded kanban merge is added (cron/webhook don't do that either) —
that query-budget optimization is deferred as a separate cross-cutting change.
The pinned-survives-hide finding is also out of scope (cron/webhook hide pinned
identically, confirmed by Codex). Adds bite-tested regressions.

Co-authored-by: vduke <vduke@users.noreply.github.com>

---------

Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>

api/route_session_list_cache.py

@@ -127,6 +127,7 @@ def _session_list_cache_key(
     exclude_hidden: bool = False,
     visible_only: bool = False,
     show_webhook_sessions: bool = False,
+    show_kanban_sessions: bool = False,
     source_filter: str | None = None,
     sidebar_source: str | None = None,
     archived_limit: int | None = None,
@@ -152,6 +153,7 @@ def _session_list_cache_key(
         bool(exclude_hidden),
         bool(visible_only),
         bool(show_webhook_sessions),
+        bool(show_kanban_sessions),
         source_filter,
         sidebar_source,
         normalized_archived_limit,