feat(seer): Add feature flag for thinking blocks toggle

Register organizations:seer-explorer-thinking-blocks, which will gate the
thinking-blocks toggle in the Seer Explorer top bar. The frontend change
to read the flag ships in a separate PR.

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

Agent transcript: https://claudescope.sentry.dev/share/nwjiQTuoXn_gjAbTTeEIIVkV9-uR45bdqtilj4y7vEM

Author: chromy

src/sentry/features/temporary.py

@@ -324,6 +324,8 @@ def register_temporary_features(manager: FeatureManager) -> None:
     manager.add("organizations:seer-explorer-chat-coding", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable code mode tools (sentry_api_search/execute) in Seer Explorer
     manager.add("organizations:seer-explorer-code-mode-tools", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
+    # Enable the thinking blocks toggle in the Seer Explorer top bar
+    manager.add("organizations:seer-explorer-thinking-blocks", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable structured LLM context (JSON snapshot) instead of ASCII DOM snapshot
     manager.add("organizations:context-engine-structured-page-context", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Allow root_cause as a valid automated run stopping point and org-level default