Session artifacts UX - tracker issue

[romalpani]

Session artifacts are things generated by the agent during a session, potential next actions, and a quick glance summary of agents work.
Reference: https://vscodeteam.slack.com/archives/C1C2M1KEZ/p1786930229861589

1. Session artifact pills - Show references as a separate item in the response body/footer, instead of as an artifact pill #333068
2. Session artifact pills - Add background and fading to have clear separation between background text and pills #333069
3. Session artifacts UX - Convert comments, pipeline failures into alert pills (leverage sub-agent alert state UI for this) #333070