pi/omp: discover transcripts whose session record follows a title slot

[avs-io]

This overlaps #846, which came first. @jbspeakr — who reported #845 — opened #846 fixing the same defect. This branch was built independently from the issue before #846 surfaced in a PR sweep. #846 should have precedence.

Left open as a draft only for the one difference described below. If that difference is not wanted, close this in favour of #846.

Relates to #845.

The defect

The shared Pi/OMP JSONL discovery gate inspected only the first physical line of a transcript for a type: "session" record. Oh My Pi writes a fixed-width type: "title" metadata line ahead of that header, so valid OMP transcripts were excluded from codeburn sessions --provider omp. Both this branch and #846 scan past leading non-session lines to fix it, and both keep Pi and OMP on the identical shared path.

Difference from #846

Read cost. #846 keeps readSessionFile, which reads the entire transcript into memory (up to the 128MB cap) and then iterates its lines. This branch streams through readSessionLines — the helper codex.ts, droid.ts and others already use — and stops after 20 leading lines.

The distinction matters most for the negative case. A message-only file with no session record is read in full under both the old code and #846; under a bounded scan it costs 20 lines. Session directories accumulate such files, and discovery walks all of them on every run.

Worth noting the pre-existing code only looked bounded: it read the whole file and then used line 0. So this is not a regression #846 introduces, it is an existing cost neither the old code nor #846 avoids.

One semantic change here: readFirstEntry now returns only session entries rather than whatever line 0 parsed to. The caller's first.type !== 'session' guard becomes redundant but is left in place rather than widening the diff.

Acceptance criteria

• OMP title-first JSONL transcripts are discovered
• Blank lines before the session record do not prevent discovery
• Message-only files with no session record remain excluded
• Pi and OMP retain identical shared discovery behaviour

Testing

Fixture tests in tests/providers/pi.test.ts reproducing a title-slot-first OMP transcript, confirmed failing before the change. Also covered: blank-line tolerance, malformed leading JSON handled without throwing, message-only exclusion, a session record placed beyond the scan bound (which must not be found, proving the scan is bounded rather than full-file), and a large message-only file completing well inside a generous time ceiling.

Full suite 2471 passed, tsc --noEmit clean.

Credit

@jbspeakr reported #845 and traced the format to its upstream origin (can1357/oh-my-pi@0ce330ab, 2026-06-27), which is what made this straightforward to scope.