Why

• Runtime analytics events report thread_id, which identifies the individual thread emitting an event
• They don't report session_id, which identifies the shared session for a root thread and its subagent threads
• Emitting both identifiers allows analytics to group related activity

What Changed

• Adds session_id to relevant analytics events (thread_initalized, turn, turn_steer, compaction, guardian_review)
• Tracks each thread's session ID in the analytics reducer so subsequent thread scoped events emit the same value
• Carries the shared session ID through subagent initialization

Verification

• just test -p codex-analytics validates event payloads and subagent session grouping.
• Focused codex-app-server tests validate session IDs for thread, turn, and steer events.
• Focused codex-core tests validate root and subagent session ID propagation.