v4.39.1 — same-thread merge claim: honest boundary

Highlights

• The same-thread merge claim now states what git actually guarantees. v4.39.0's docs
  said "both sides editing the same thread conflicts per-file." An independent CoPilot
  assessment, delivered the day the release shipped, flagged that as overstating git's
  hunk semantics — and live merge rehearsals confirmed it, more broadly than the
  assessment hedged: git raises a conflict only when same-thread edits touch
  adjacent/overlapping lines; edits separated by even one unchanged line merge
  cleanly, in a 5-line thread file as much as a 16-line one, with both sides kept.
• Why this is precision, not a fix. Nothing is lost in the clean-merge case —
  keep-both is MERGE.md's own Tier 1 outcome; whether the two surviving statements are
  consistent is the write-time contradiction check's job (DECAY.md §10); and the
  pre-4.39.0 layout merged the same edits identically, so there is no regression. But the
  claim must not outrun the mechanism, so the docs now state the measured boundary and
  the contract test pins it — the honest-signal doctrine applied to the tool's own
  release claims.

What's changed

• MERGE.md, .agent/schema.md, docs/DESIGN-merge-scale.md — the same-thread claim
  states the measured boundary (adjacent/overlapping → conflict, the Tier 2 human gate;
  separated → clean keep-both → run the contradiction check on the merged thread).
• tests/test_thread_layout_merge.sh — two new cases pin the boundary with real git
  merges: separated same-thread edits merge clean with both edits present (documented
  behavior, not a bug), adjacent edits conflict. 4 → 6 cases, all green.
• Lockstep: VERSION 4.39.1, CHANGELOG.md, UPGRADE.md ladder row +
  4.39.0 → 4.39.1 rung, README release row.
• Zero behavior change: no scripts, no template protocol, no memory-file shape.

Upgrading an enabled repo

Nothing to schedule: this is a plain re-copy of MERGE.md + .agent/schema.md
(verbatim rows), which the next reconcile of any repo performs automatically — no
semantic step, no adapter re-sync, no migration. Say "AI enable this repo" whenever
convenient, or let it ride along with the next upgrade that touches the repo anyway.

Full details

• CHANGELOG.md — Version 4.39.1
• UPGRADE.md — the 4.39.0 → 4.39.1 rung
• docs/DESIGN-merge-scale.md — the measured boundary, recorded where the design claim lives