feat(annotate): per-file version diff for .md and .html (rendered HTML highlights)

[egouilliard-leyton]

Closes #960

What

Brings the version diff (the highlighted "what changed since the last version" view that plan mode has) to annotate mode, for both Markdown and HTML files. Annotate already has the diff UI (PlanDiffViewer, Version Browser, block-level comments) — it just never received previousPlan/versionInfo, because the annotate server didn't track history. This wires that up.

• .md / .txt → the existing rendered Markdown diff (clean / classic / raw), with block comments.
• .html with --render-html → the diff is rendered as the real page with inline <ins>/<del> highlights (not a source dump), so you see where it changed, and commenting still works because it's the same HtmlViewer iframe.

How

• History keyed by file path. Unlike the plan flow (generateSlug = first-heading + date), annotate keys history by file path — annotate-{sanitized-basename}-{hash8} — so re-opening the same file groups its versions across edits even when headings change. Dedup + sequential numbering reuse the existing saveToHistory storage.
• Server (packages/server/annotate.ts + Pi apps/pi-extension/server/serverAnnotate.ts). On open, save the markdown (or raw HTML source) to history, then expose previousPlan / versionInfo / diffCurrent on /api/plan and add /api/plan/version + /api/plan/versions. Both runtimes updated per AGENTS.md.
• HTML rendered diff. New packages/shared/html-diff.ts: a small, dependency-light, tag-aware htmlDiff(old, new) that wraps changed text in <ins>/<del> while keeping tags balanced (<script>/<style> treated as opaque). The server sends diffHtml = rewriteHtml(htmlDiff(prev, current)); HtmlViewer injects ins/del highlight CSS and shows a "Show changes" toggle; App swaps the iframe to the diff page when toggled and suppresses the Markdown block-diff path on the HTML surface (where markdown is empty).

Testing

• packages/shared/html-diff.test.ts — 9 unit tests (text change, word replace, added element, script/style opacity, identical-input no-op, structural balance).
• tsc --noEmit clean for packages/shared, packages/server, packages/ui; frontend build clean.
• Manually verified end to end on a real 58 KB HTML spec: the diff renders as the page with inline highlights, the toggle works, and commenting works on the diff view. /api/plan returns renderAs: html + diffHtml, and /api/plan/versions lists the stored versions.

Notes

• HTML diff is a linear token diff (delete + insert for moves; attribute-only changes re-emit the tag without a visible marker). Good enough for the prose/spec-review use case; a tree-aware pass could be a follow-up.

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