Agent identity: Agents that act and appear as themselves

What this post added

This post introduces agent identity as a core component of Glean's independent agents, allowing agents to act through their own service account credentials. This decouples agent invocation from user access, provides agents with their own presence across surfaces (Slack, Jira, GitHub), ensures agents are trusted with exactly what the job needs via scoped credentials, and enables scheduled work that doesn't silently expire due to user token expiration. Actions are properly attributed to the agent's own account in the audit trail.