feat(skills): Enforce PR assignment gates and stream metadata via stdout

Verify pull request assignment against the active GitHub user to prevent
concurrent triage overhead and restrict local checkout to the assigned owner.

Additionally, stream the fetched PR metadata directly via stdout instead of
writing to a local workspace cache file to prevent disk clutter and simplify
assistant JSON parsing.

Change-Id: I9408fba5c200cca5814afb7223302eb849c0b319