#4158 Agent sessions extension: import Codex sessions

Part of agent observation (scope doc: https://claude.ai/artifact/NdQUomjuVAfo4SsPuRWYWk; harness survey #4117, OTEL research #4118). Agent sessions show what a coding agent did in a swamp repo: its turns, its tool calls, and whether each call used swamp, went around it (a bypass), or neither, with findings that say what to tune.

How it ships. The data comes from the harnesses' own local session files (Claude Code transcripts, Codex rollouts), which need no hooks or setup. An unpublished extension imports them as ordinary repo data. A core dashboard page shows them, hidden unless the repo has session data the viewer can read, with a Preview chip. No flag: until the extension is published, nobody else gets data, so nobody sees the page.

Reference prototype (local, private, not for sharing): ~/scratch/agent-sessions-prototype/ on Seth's machine. See its README.

Repo: swamp-extensions, in the extension from #4156.

Goal

Import Codex sessions into the same schema (#4154), so the page and classifier (#4157) work unchanged. This is the first test that the schema really is harness-neutral; feed anything that doesn't fit back into #4154.

What we know

Codex writes rollouts to ~/.codex/sessions/YYYY/MM/DD/rollout-*.jsonl. Record types seen:

• session_meta and turn_context: session and cwd;
• event_msg task_started and task_complete: turn boundaries;
• response_item custom_tool_call and custom_tool_call_output: tool calls (there may be other call types such as function_call);
• event_msg token_count and token_usage_record: tokens.

Only two short sessions exist locally so far; run a few real Codex sessions in a swamp repo first to get realistic fixtures.

Scope

• Map the records to session, turns, calls and tokens; find the sessions for the current repo by cwd.

• Same rules as #4156: no prompts or replies, the same redaction, re-import replaces.

• Map Codex's shell tool so #4157's segment rules apply.

• Find out whether Codex runs subagents or parallel agents and how they appear in rollouts; import them at full detail, linked to their parent, as #4156 does for Claude Code.

Out of scope

OpenCode: a follow-up issue once there are real OpenCode sessions to work from.