story-kit: create 89_story_persistent_per_session_agent_logs

.story_kit/work/1_upcoming/89_story_persistent_per_session_agent_logs.md

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+---
+name: "Persistent per-session agent logs"
+test_plan: pending
+---
+
+# Story 89: Persistent per-session agent logs
+
+## User Story
+
+As a user, I want each agent session to write its output to a persistent log file so I can inspect what an agent did after it completes, even across server restarts.
+
+## Acceptance Criteria
+
+- [ ] Each agent session writes output to a log file at .story_kit/logs/{story_id}/{agent_name}-{session_id}.log
+- [ ] Log files persist across server restarts and agent completions
+- [ ] The get_agent_output MCP tool falls back to reading the log file when the in-memory stream is empty or the agent has completed
+- [ ] Log files include timestamps, tool calls, text output, and status events
+- [ ] Different sessions for the same agent on the same story produce separate log files (no mixing)
+
+## Out of Scope
+
+- TBD