feat(core): replace webfetch markdown renderer - #42229

What

Replace V2 Core webfetch's Turndown/Domino HTML-to-Markdown conversion with a purpose-built htmlparser2 event renderer. The renderer preserves model-readable document structure without constructing a browser DOM and removes Core's turndown, @mixmark-io/domino, and @types/turndown closure.

How

• packages/core/src/tool/html-markdown.ts implements an iterative semantic frame renderer for headings, paragraphs, inline formatting, links/images, blockquotes, nested lists, inline/fenced code, and readable tables.
• Active/fallback elements are suppressed, entities decode through htmlparser2, code fences expand around embedded backticks, and preformatted content bypasses whitespace normalization.
• List/quote indentation is capped and pathological open-tag depth takes a deterministic text-only path, bounding amplification for webfetch's 5 MiB response limit.
• packages/core/test/tool-webfetch.test.ts covers the public helper and tool path with malformed/deep input, Unicode/entities, active content, whitespace, code, nested lists/tables, and output-growth regressions.
• packages/core/perf/html-markdown.md records the autoresearch experiments and measurements.

Scope

• V2 only: Core and the V2 CLI artifact were changed/evaluated.
• packages/opencode is untouched.
• No public package or API was added; WebFetchTool.convertHTMLToMarkdown remains the public helper.
• Base Turndown flattened table cells; the replacement intentionally emits readable GFM-style tables.
• Malformed overlapping inline/block tags may drop formatting delimiters while preserving visible text.
• Documents deeper than 10,000 open tags fall back to deterministic visible-text extraction to protect memory.
• This repository does not use Changesets, so no changeset is included.

Testing

• bun run test tool-webfetch.test.ts from packages/core: 26 passed.
• bun typecheck from packages/core: passed.
• bun typecheck from packages/cli: passed.
• Push hook bun turbo typecheck: 29/29 package tasks passed.
• MODELS_DEV_API_JSON=../core/test/plugin/fixtures/models-dev.json bun run build --single --skip-install from packages/cli: passed.
• Temporary real-site differential corpus: Example Domain, MDN table reference, Python asyncio docs, RFC 9110, and W3C forms tutorial. Snapshots were not committed.
• Full Core suite: 1,032/1,033 passed. The unrelated macOS watcher test reports .git/HEAD as add instead of expected change; it reproduces in isolation.
• Four independent review rounds found and verified fixes for depth/output amplification, fence crashes, code whitespace mutation, Markdown escaping, quote/table containment, emphasis whitespace, and nested tables.