[hdamker]

What type of PR is this?

• cleanup

What this PR does / why we need it:

Removes the release_plan_change path, which can never fire (see #483): push payloads carry no per-commit file list, so every push is classified as push.

Removed:

• the release_plan_change trigger type in trigger.py; a push is now always trigger_type: push, command: sync-issue
• the config_drift_warning template and its workflow step
• the release-plan line of issue_created and the trigger_release_plan_change flag
• the merged-PR lookup for push events (GitHubClient.find_merged_pr_for_commit) and the trigger_pr_number / trigger_pr_url outputs and context fields. Their only consumers were the two removed lines; the lookup cost one API call per push
• the matching lines in technical-architecture.md; the drift-warning paragraph now states what holds instead: an active snapshot carries its own release-metadata.yaml, validation blocks release-plan.yaml edits while a snapshot is active, and only /create-snapshot is gated on the common-file cache check

No visible change: the comment, labels and Release Issue sections a push produces are the same as before (a test pins the issue_created text after a push).

Which issue(s) this PR fixes:

Fixes #483
Part of #476

Special notes for reviewers:

Targets release-automation-refactor. The branch canary on ReleaseTest2 follows after merge.

Changelog input

 release-note

Additional documentation

docs