Simulates the SessionStart warning added in session-registrar.ts (#2242). The warning fires
only on the genuinely dangerous case: a live peer sharing repo + branch + the same main working tree.
status='running' · same repo_hash · same branch ·
worktree_path IS NULL · sid != self · last_heartbeat > now-300s
— AND this session is also on the main tree (worktree_path IS NULL) with a non-null branch.