CHANGELOG.md: 161 lines of entries sit above the file title, with a duplicate `## [Unreleased]`

[seonghobae]

What is wrong

CHANGELOG.md on main is structurally malformed. Heading map of the first 200 lines:

  1:### Failed-check finding names the Strix sandbox instead of the gateway
  5:### Strix gate keeps a recovered transient model error from failing a completed scan
  9:### Review sidecar preflight postpones a rate-limited account's candidates ...
 ...   (18 more ### entries)
 70:## [Unreleased]          <-- first Unreleased
155:## 2026-09-02 — Noema single-request gateway ownership
162:# Changelog               <-- the file title, at line 162
170:## [Unreleased]          <-- second Unreleased

Three separate defects, all from the same cause:

1. The file title # Changelog is at line 162, not line 1. Everything above it is orphaned content with no document root.
2. There are two ## [Unreleased] sections (lines 70 and 170).
3. A dated release section (## 2026-09-02, line 155) is stranded between them, above the title, so it reads as newer than releases that are actually newer.

Cause

Successive PRs have each prepended their entry at line 1 rather than under the real ## [Unreleased] at line 170. Each prepend was individually reasonable — the top of the file looked like where recent entries go, because the previous prepend had put one there. Nothing catches it: no test parses CHANGELOG.md's structure. tests/test_agent_review_runtime_quality_consolidation.py and tests/test_pr_review_autofix_nvidia_nim_contract.py reference the file but match prose, not headings.

Why it matters beyond tidiness

The changelog is an acquisition-readiness artifact, not just developer notes. A buyer reading it top-to-bottom sees 161 lines before the document says what it is, then a title, then a second "Unreleased" heading. It also makes every future prepend ambiguous, so the defect keeps growing.

Proposed repair

One dedicated PR, no behavior change:

1. Move # Changelog (and any preamble) to line 1.
2. Merge the two ## [Unreleased] sections into one, preserving entry order (the orphaned block at lines 1–69 is newest, so it leads).
3. Move the ## 2026-09-02 dated section below ## [Unreleased], into date order with the other dated sections.
4. Add a contract test asserting the invariants that would have caught this: exactly one # heading and it is the first heading in the file; exactly one ## [Unreleased]; it precedes every dated ## section; dated sections are in descending date order.

Step 4 is the part that matters — without it the same drift resumes on the next prepend.

Sequencing

This touches the top of a file that nearly every open PR also touches, so it will conflict with all of them. It should land when the open-PR queue is shallow, and the repair PR should be rebased-and-merged quickly rather than left open. Deliberately not folded into #2137 (orchestrator pin advance) for that reason; #2137's own entry follows the current (wrong) convention so it sits with its peers and this repair stays one coherent change.

Found while placing the changelog entry for #2137.