feat: extract and surface Dev Mode annotations

[qqqq5953]

Motivation

ROADMAP.md already listed this as an open item: "Investigate feasibility of pulling in annotations via the Figma API." This PR implements it.

Background

@figma/rest-api-spec (v0.37.0) publishes AnnotationsTrait as an empty object — the annotations field on nodes isn't typed at all in the generated spec. However, the live Figma REST API does return it on nodes that carry a Dev Mode annotation. I verified this against a real Figma file with an existing annotation — the raw GET /v1/files/:key/nodes response actually contains "annotations": [{ "label": "..." }] on the relevant node — so this isn't based on documentation alone, it's confirmed against live API responses.

What changed

• src/transformers/annotation.ts (new): models the real wire shape (label, labelMarkdown, properties[].type, categoryId) since the spec package doesn't, and simplifies it to { label?, properties?: string[], categoryId? } — preferring labelMarkdown over label when both are present, and dropping the property wrapper objects for token efficiency.
• src/extractors/built-in.ts: new annotationExtractor, added to allExtractors so it's included by default in get_figma_data output.
• src/extractors/types.ts: added annotations?: SimplifiedAnnotation[] to SimplifiedNode.
• src/utils/serialize-tree.ts: added annotations= rendering to the default tree output format.
• Wired through the src/extractors/index.ts / src/index.ts export barrels, updated src/extractors/README.md, and checked off the corresponding ROADMAP.md item.

Testing

• Added unit tests for annotationExtractor (label/labelMarkdown precedence, pinned properties, omission when absent) in src/tests/tree-walker.test.ts.
• Added a regression test in src/tests/serialization.test.ts for the tree-format serializer — the field is easy to add to the extractor/type and forget to wire into the tree renderer's field whitelist, which is exactly what happened during my own testing before I caught it.
• Manually validated end-to-end against a live Figma file with an existing Dev Mode annotation: raw REST API response → simplifyRawFigmaObject → serializeResult(..., "tree"), confirming the annotation text survives all the way to the actual string the get_figma_data tool returns.
• pnpm test, pnpm type-check, pnpm lint, pnpm prettier --check, and node scripts/scan-hidden-chars.mjs all pass.

Scope note

This only covers reading annotations (surfacing them in get_figma_data output), matching this project's read-only REST API architecture. There's no REST endpoint to write annotations back — only the Figma plugin API supports that — so this PR doesn't attempt it.