bug: buildSessionMeta preserves stale entry_count when meta.json exists

[galexy]

Symptom

ox session upload's buildSessionMeta reads any existing meta.json and reuses it as-is — including the entry_count field. It never recomputes entry_count from the actual raw.jsonl on disk. So any session whose raw.jsonl changed size after meta.json was first written reports a wrong count forever (or until meta.json is manually deleted).

Evidence

During the #519 investigation:

1. ox agent <id> session import wrote meta.json with entry_count: 50 (the 50 entries it imported).
2. raw.jsonl was rebuilt in place to 235 entries.
3. ox session upload ran. Preserved entry_count: 50 in meta.json even though raw.jsonl now had 235 entries.
4. Dashboard and ox session list reported 50 entries for a 235-entry session, long after upload.

The stale count persisted through several upload attempts until meta.json was explicitly deleted before upload.

Root cause

cmd/ox/session_upload_cmd.go:141-177:

func buildSessionMeta(sessionPath, sessionName, projectRoot string, fileRefs map[string]lfs.FileRef) (*lfs.SessionMeta, error) {
    // try reading existing meta.json first
    meta, err := lfs.ReadSessionMeta(sessionPath)
    if err == nil {
        // update the file manifest
        if fileRefs != nil {
            meta.Files = fileRefs
        } else if meta.Files == nil {
            meta.Files = make(map[string]lfs.FileRef)
        }
        return meta, nil
    }
    // ... fallback: count entries from raw.jsonl, build fresh meta
}

Only Files is updated on the "existing meta.json" path. entry_count, summary, and other fields are preserved as-is. The fallback branch (when no meta.json exists) correctly calls countJSONLLines(filepath.Join(sessionPath, ledgerFileRaw)), but it only runs when there's no existing meta.

Impact

• Any session re-uploaded after its raw.jsonl changed size publishes wrong metadata.
• ox session list and the dashboard show stale entry counts.
• Obscures whether a session was modified or re-captured — users can't tell from metadata alone.
• Downstream tooling (summaries, stats, session filtering) that trusts entry_count is silently wrong.

Fix direction

buildSessionMeta should always recompute entry_count from raw.jsonl on disk, regardless of whether existing meta.json had a value. It's a cheap file-line count — no reason to preserve the stale number.

Consider generalizing: any field in meta.json that's derivable from content files (entry_count, summary) should be recomputed at upload time, not preserved.

Acceptance

• Modifying raw.jsonl and running upload writes a meta.json with the correct line count.
• A test fixture exercises: existing meta.json with entry_count=N, modify raw.jsonl to have M entries, call upload, assert meta.json now reports M.

Related

• Sibling issue: ox session upload replaces content files with LFS pointers before reading them — same class of "trust existing meta.json too much" bug.
• 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.