Organization agent sessions page contract

Cross-repo browser of captured agent sessions for one organization, restricted to repositories the current user can read. Returns the filtered/paginated session list with per-session stats, filter facets, and — when session is supplied — the expanded session's checkpoint timeline, its server-derived git commits and pull_requests, and its checkpoint linked_commits. Each expanded checkpoint carries the Git commit IDs it is attached to (linked_commits), resolved through the reverse checkpoint→commit lookup; linked_commits_available is false and the lists are empty when that lookup is unavailable. Payloads carry sanitized provenance only (no storage keys, checksums, or archive URLs).