fix(channels): require bound identity for user-owned IM messages (#3578)

* fix(channels): require bound identity for user-owned IM messages

* make format

* docs: document bound identity channel config

* refactor: reuse channel connection config

* refactor _requires_bound_identity()

* refactor from_app_config()

* make format

* fix: reject unbound channel chats before semaphore

* security enhancement

* make format

* fix: enforce bound-identity admission at command entry point

The bound-identity gate only ran for non-command messages in
_handle_message() and as a fallback inside _handle_chat(). Commands had
no equivalent boundary, so an unbound platform user could send /new and
reach _create_thread() directly, creating an unowned Gateway thread and
empty checkpoint. Info commands (/status, /models, /memory) likewise
leaked Gateway state to unbound users.

Add the same _requires_bound_identity() check at the top of
_handle_command(), rejecting via _reject_unbound_channel_message() before
any thread creation or Gateway query. The gate is a no-op in legacy
open-bot mode (require_bound_identity=False) and auth-disabled mode.
Provider-level binding flows (/connect, /start) are consumed by the
provider adapter before reaching the manager, so they are unaffected.

Tests:
- unbound auth-enabled /new is rejected before threads.create
- bound auth-enabled /new still creates the thread

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(channels): carry workspace fallback decision on inbound messages

* fix(channels): recheck bound identity by normalized workspace

* fix(channels): avoid duplicate bound identity checks

* fix(channels): preserve verified routing for bound identity rejects

* fix(channels): clarify bound identity upgrade failures

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

config.example.yaml

@@ -15,7 +15,7 @@
 # ============================================================================
 # Bump this number when the config schema changes.
 # Run `make config-upgrade` to merge new fields into your local config.yaml.
-config_version: 13
+config_version: 14
 
 # ============================================================================
 # Logging
@@ -1208,6 +1208,9 @@ run_events:
 #
 # channel_connections:
 #   enabled: false
+#   # Security: keep this enabled unless you intentionally want legacy open-bot behavior.
+#   # Disabling it lets unbound external IM users create DeerFlow threads/runs.
+#   require_bound_identity: true
 #
 #   telegram:
 #     enabled: false