bug: regenerateArtifacts silently skips summary.md when summary.json is missing

[galexy]

Symptom

ox session regenerate <name> (default mode) regenerates markdown artifacts from a session's raw.jsonl. session.md gets regenerated unconditionally, but summary.md is only written if summary.json already exists in the session directory. If summary.json is missing, summary.md is silently skipped — no warning, no fallback template.

Evidence

Reproduced during #519 investigation. Recreated a session in cache with only raw.jsonl, ran ox session regenerate <name>. Output claimed success:

✓ Regenerated artifacts for 2026-04-19T05-59-galexy-OxBkup

But on disk:

-rw-r--r--  raw.jsonl         250496 bytes
-rw-r--r--  session.md        100428 bytes   ← regenerated
                                            ← no summary.md

No warning that summary.md wasn't written. Only session.md was produced.

Root cause

cmd/ox/session_regenerate.go:691-706:

// summary.md — regenerate from summary.json if available
summaryJSONPath := filepath.Join(sessionPath, "summary.json")
if data, err := os.ReadFile(summaryJSONPath); err == nil {
    var summaryResp session.SummarizeResponse
    if json.Unmarshal(data, &summaryResp) == nil {
        summaryView := session.SummarizeResponseToSummaryView(&summaryResp)
        summaryMdGen := session.NewSummaryMarkdownGenerator()
        summaryMdBytes, err := summaryMdGen.Generate(rawSession.Meta, summaryView, rawSession.Entries)
        if err == nil {
            summaryMdPath := filepath.Join(sessionPath, ledgerFileSummaryMD)
            if writeErr := os.WriteFile(summaryMdPath, summaryMdBytes, 0644); writeErr != nil {
                errs = append(errs, ...)
            }
        }
    }
}

Three silent-skip points:

1. summary.json doesn't exist → outer if skips. No fallback.
2. summary.json fails to unmarshal → inner if skips. Error not collected.
3. summaryMdGen.Generate fails → skip. Error not collected.

Only write errors get added to errs and surfaced to the user.

Impact

• Users rebuilding a session from raw.jsonl (e.g., recovering from data loss, importing prior history) get session.md but no summary.md. They don't know this unless they check.
• To get summary.md, users must either have an existing summary.json or run ox session regenerate --summary — which invokes claude as a subprocess (requires the CLI, uses LLM tokens, may take minutes).
• There's no template-based fallback for summary.md even though session.md has a perfectly good template-based generation path (NewMarkdownGenerator).
• Contrast with ox agent session import (cmd/ox/agent_session_plan_history.go:148-157) which DOES generate a template-based summary.md without an LLM:

summaryMDGen := session.NewSummaryMarkdownGenerator()
summaryBytes, summaryErr := summaryMDGen.Generate(stored.Meta, nil, stored.Entries)
//                                                             ^^^ nil SummaryView works

The generator accepts a nil SummaryView and produces a structural summary.md from meta + entries alone. regenerate gates on a non-existent summary.json for no real reason.

Fix direction

In regenerateArtifacts, always generate summary.md:

1. If summary.json exists and parses → generate with the rich summary view.
2. Otherwise → generate with nil SummaryView (structural fallback). Same behavior as session import.

Also surface the three silent-skip points as warnings at minimum — users should know when a file they expect didn't get written.

Acceptance

• Running ox session regenerate <name> on a session with no summary.json produces a non-empty summary.md.
• The output of regenerate either reports which artifacts were written or warns which were skipped — no silent partials.
• Test fixture covers: session dir with only raw.jsonl → both session.md and summary.md produced.

Related

• Discovered during bug: session recording produces empty raw.jsonl (header only, 0 turns captured) on macOS / Claude Code 2.1.112 / ox 0.6.3 #519 investigation while rebuilding a session from scratch.