fix: parse README pin and changelog as markdown

[John-David Dalton (jdalton)]

Port of the v1.x AST-markdown rework (landed there as 6e5adcb) to main's fleet shape: markdown structure questions are answered from a position-tracked GFM mdast parse instead of line scans and string patterns, so content inside code fences can never read as structure.

What ported where (main's surfaces differ from v1.x):

v1.x surface	main surface	Change
.github/scripts/pin-readme-assets.mjs (standalone script)	scripts/fleet/publish-infra/pin-readme.mtspinReadmeAssets() (pure function inside the pin/restore bracket)	Rewrite ported: mdast image/link/definition urls + parse5-located src/srcset values, edits applied at parser-reported byte offsets, no serializer round-trip. Signature unchanged.
scripts/release/changelog.mts	scripts/fleet/lib/changelog-render.mtsunreleasedRange() + scripts/fleet/lib/changelog.mtssectionHasEntries() and mergeUnreleased()'s insertion scan	Same port: ## boundaries from heading node positions, entries from real listItem nodes. Exported APIs unchanged.
(new)	scripts/fleet/_shared/markdown-ast.mts	One shared GFM parse helper (parseMarkdownGfm, h2LineIndexes, hasListItem) so the pin and the changelog flows read the same tree.

Behavior fixed on main by the port, each with a test: assets/ inside fenced code blocks or inline code spans stays as written; srcset attributes are now pinned (the string version only handled src=/](); reference-style definitions are now pinned; a ## line inside a code fence can no longer truncate a promoted [Unreleased] block or misplace the merge insertion point; a - lookalike in fenced shell output no longer satisfies the empty-changelog guard. Differential receipt: with the port reverted (tests kept), 10 of the 18 new tests fail; with the port, 18/18 pass.

Dependencies: mdast-util-gfm@3.1.0, micromark-extension-gfm@3.0.0, and parse5@8.0.1 added to the pnpm catalog + root devDependencies (mdast-util-from-markdown, micromark, and @types/mdast were already in the catalog).

Cascade note:scripts/fleet/** is managed by the shared fleet template. This lands the fix downstream to stop the bleeding; the same change needs a matching upstream template pass (including parseSectionBullets, which keeps a line loop here since it only sees generated sections) or the next cascade reverts it.

Tests: 13 in test/fleet/pin-readme.test.mts, 5 in test/fleet/changelog-markdown-structure.test.mts.

Suite: pnpm test --all locally — 7552 passed, 425 skipped, 8 failed. All 8 failures are cmd-optimize integration snapshot mismatches (packages/cli/test/integration/cli/cmd-optimize.test.mts, cmd-optimize-output-and-paths.test.mts) that fail identically on an untouched origin/main checkout (verified by stashing this branch's changes and rerunning both files) — pre-existing local-environment snapshot drift, not from this diff. pnpm run check passes on the modified scope except the pre-existing dispatch-table-is-current failure, which also fails on untouched origin/main.

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Note

Medium Risk
Touches release changelog promotion/merge and publish-time README rewriting; behavior changes are intentional but could affect edge-case CHANGELOGs or READMEs that relied on the old heuristics.

Overview
Replaces line scans and regex with a shared GFM mdast parse (scripts/fleet/_shared/markdown-ast.mts) so markdown inside code fences is never treated as structure.

Publish README pin (pinReadmeAssets): relative assets/… refs are rewritten at parser byte offsets (markdown image/link/definition plus parse5-located src/srcset), with no serializer round-trip. Adds coverage for srcset, reference definitions, and skipping lookalikes in fences or inline code.

Changelog flows: [Unreleased] section bounds, merge insertion, and sectionHasEntries use real ## headings and listItem nodes instead of startsWith('## ') / /^\s*-\s/ patterns.

Adds catalog deps mdast-util-gfm, micromark-extension-gfm, and parse5, plus fleet tests for pin and changelog structure.

^{Reviewed by Cursor Bugbot for commit 227a305. Configure here.}