story-kit: create 93_bug_agent_session_id_is_null_while_running

.story_kit/work/1_upcoming/93_bug_agent_session_id_is_null_while_running.md

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+---
+name: "Agent session_id is null while running"
+---
+
+# Bug 93: Agent session_id is null while running
+
+## Description
+
+The session_id from Claude Code's initial `system` JSON event is captured during PTY streaming (agents.rs:2011) but only stored on the agent record after the process exits (agents.rs:1293 in run_server_owned_completion). While the agent is actively running, list_agents and get_agent_output always report session_id: null.
+
+## How to Reproduce
+
+1. Start an agent on any story via start_agent
+2. Call list_agents while the agent is running
+3. Observe session_id is null
+
+## Actual Result
+
+session_id is null for the entire duration of the agent run. Only populated after the process exits.
+
+## Expected Result
+
+session_id should be set on the agent record as soon as the first `system` JSON event is parsed from the PTY stream, making it available for live log retrieval and monitoring.
+
+## Acceptance Criteria
+
+- [ ] list_agents returns a non-null session_id within seconds of agent startup
+- [ ] session_id is available for persistent log lookup while agent is still running