fix: changelog extractor drops redundant version-heading lines (audit P2-7)

[abrichr]

Context: the docs-audit P0/P1 findings were already resolved on main

This branch started from a docs.openadapt.ai audit (docs-findings.md, IDENTIFICATION pass) that flagged P0/P1 issues. On investigation, that audit was run against a stale checkout: it references docs/packages/openadapt-ml.md, -wright, -evals, architecture.md, and a package-first nav that no longer exist on origin/main. The current site has since been rewritten to a hand-authored, flow-first, product-first structure, and every P0/P1 is already fixed there:

• P0-1 (flow absent): openadapt-flow is in repos.yml (core/beta) and featured as the "Canonical compiler and governed runtime" in docs/ecosystem/index.md; the entire index.md, concepts/, and reference/cli.md ("openadapt flow CLI") are about it.
• P0-2 (superseded ML thesis): index.md already leads with the demonstration-compiler framing ("Show it a repeated workflow. OpenAdapt compiles it into governed, deterministic replay") — deterministic model-free replay, effect verification, halt-don't-guess, all substrates, local-first + Cloud.
• P1-3 / P2-5 (broken images/links in synced package pages): those packages/*.md mirrors were deliberately retired. repos.yml sets no doc_page, sync_readmes.py therefore syncs nothing, and validate_docs.py + test_sync_readmes.py now enforce the page-free, no-package-first-nav contract. Re-introducing README-mirroring (and the requested sync_readmes.py path-rewrite) would regress the site and break those guards, so it is intentionally not done. The old packages/openadapt.md is a redirect stub to /ecosystem/.
• P2-6 / P2-8 (whats-new / architecture.md stale/orphaned): whats-new.md regenerates on the sync.yml cron (last updated today); architecture.md was removed in favor of concepts/.

What this PR changes: the one genuine residual owned-file bug (P2-7)

aggregate_changelog.py rendered each release as the tag line plus the release body's first line. GitHub release bodies open with a heading that only restates the version (## v1.7.1 (2026-07-19)), so the live changelog.md echoed a duplicate ## vX heading with no notes, also corrupting the page's heading hierarchy.

• Add summarize_body(): skip a leading heading that merely restates the version, strip leading # so the first real note renders inline, and emit nothing when the body has no notes beyond the version header.
• Widen the fetched body window (200 → 400 chars) so real notes below the header survive truncation.
• 4 new tests (unit + end-to-end asserting no ## vX heading leaks into the changelog).

The sync.yml cron runs aggregate_changelog.py, so the live changelog picks this up on the next sync. changelog.md itself is not hand-edited (it is bot-regenerated), per the "do not hand-fake changelog content" instruction.

Verification

• pytest tests/ — 68 passed (was 64; +4).
• python scripts/validate_docs.py — Validation passed.
• mkdocs build --strict — exit 0.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NyCHrzA1psrKMFfroYbzaM