#4328 agent-sessions: low-severity edge cases from the Codex importer reviews (#4158)

Follow-up to #4158 (Codex import in @swamp/agent-sessions, agent-sessions/ in swamp-extensions). Three verify-reviews rounds passed; the medium findings were fixed in the PR. These lows were left for one follow-up:

Root and error handling

• An unreadable Claude Code root (PermissionDenied, not NotFound) still stops the Codex import; the Codex root was made a warning, the Claude root was not.
• With no HOME or CLAUDE_CONFIG_DIR, an import that passes codexDir is rejected for lack of a Claude root.
• An explicit codexDir that does not exist is silently ignored; only an explicit one should warn.
• An unexpected error in Codex discovery (other than an unreadable root) still aborts Claude Code's import; isolate per harness.

Codex rollouts

• A session_meta first line over 4 MiB is treated as not a rollout: the session is dropped with no warning and its index entry pruned.
• A subagent rollout that is briefly unreadable drops that child and deletes its stored record as stale; the next import recreates it. Keep unreadable child ids out of the stale-child delete.
• Subagents of a top session that exists but cannot be placed are not counted as unresolved.
• A completed CommandExecution with no exit_code is recorded as failed.
• commandLine joins a non-shell argv without quoting.
• Call ids are cut to 200 characters: a cut spawn_agent call id would no longer link to its subagent, and two FileChange ids could collide. Real ids are about 40 characters.
• If a subagent's parent rollout is missing, its parentSessionId and the link lookup can pick different fallback parents.

Claude Code

• NotebookEdit now labels by notebook_path before file_path (the field its stored text already used); only differs when both are present.