fix(tool-web): bound conversion depth and complete fetch output

Two review findings on the turndown swap, both verified empirically:

- Unclosed-tag nesting makes the synchronous turndown/domino walk
  superlinear (measured: depth 512 ~0.15s, 2k ~2s, 20k ~5s), during
  which the cooperative fetchTimeoutMs timer cannot fire. renderBody
  now preflights nesting depth with a linear tag scan and passes
  bodies past 512 levels through raw; the try/catch stays for markup
  the scan cannot see (comment-hidden tags), simulated in tests via a
  converter throw.
- Markdown escaping can expand converted HTML ~2x (100k underscores
  render as 200k chars), so provider body caps no longer bounded the
  model-visible result. formatFetchOutput now caps the complete output
  (header + body + footer) under new fetchMaxOutputChars config
  (default 200000 = 2x the local provider's default body cap), reusing
  the truncation notice.

README EN+ZH, config catalog, Agent Note EN+ZH updated; the new
web-fetch fixture is migrated to the packed layout master now
requires; tool-web coverage stays 100% per-file.

.agents/notes/implemented/simplification/2026-07-26-turndown-for-tool-web-html-markdown.md

@@ -10,7 +10,7 @@ English | [中文](2026-07-26-turndown-for-tool-web-html-markdown.zh.md)
 
 ## Decision
 
-`packages/web/tool-web/src/fetch.ts` owns a module-level [`turndown`](https://github.com/mixmark-io/turndown) instance (`headingStyle: 'atx'`, `codeBlockStyle: 'fenced'`, `bulletListMarker: '-'` — fixed model-facing presentation, not deployment tunables) with `@joplin/turndown-plugin-gfm`'s composite `gfm` plugin for tables/strikethrough and `remove(['script', 'style', 'noscript'])` replacing the old wholesale drops. `renderBody`'s `html` arm calls it in a try/catch falling back to the raw HTML body: the regex version could never throw, while turndown/domino's recursive DOM walk overflows with a `RangeError` at a few thousand nesting levels (measured: 4k throws on the main thread, 8k in a worker thread), and a degraded page beats an error for a body the provider already decoded. `html.ts` and its conversion tests are deleted; the fallback and the status-header/truncation-footer formatting are tested in `tests/tool-web.spec.ts`, and the README's Known Limitations trades the regex-converter caveat for the pathological-nesting fallback. The gfm plugin ships no types; `src/turndown-plugin-gfm.d.ts` declares the one imported export over `@types/turndown` (a devDependency).
+`packages/web/tool-web/src/fetch.ts` owns a module-level [`turndown`](https://github.com/mixmark-io/turndown) instance (`headingStyle: 'atx'`, `codeBlockStyle: 'fenced'`, `bulletListMarker: '-'` — fixed model-facing presentation, not deployment tunables) with `@joplin/turndown-plugin-gfm`'s composite `gfm` plugin for tables/strikethrough and `remove(['script', 'style', 'noscript'])` replacing the old wholesale drops. `renderBody`'s `html` arm guards the conversion twice: a linear tag-scan preflight passes bodies nested past 512 levels through raw (the synchronous walk is superlinear on unclosed nesting — measured seconds at 20k levels — during which the cooperative timeout cannot fire), and a try/catch falls back to the raw HTML when turndown still throws on markup the scan cannot see; a degraded page beats an error for a body the provider already decoded. `formatFetchOutput` bounds the complete output (`fetchMaxOutputChars` config, default 200,000) because markdown escaping can expand converted HTML to ~2× a provider's body cap. `html.ts` and its conversion tests are deleted; the fallback and the status-header/truncation-footer formatting are tested in `tests/tool-web.spec.ts`, and the README's Known Limitations trades the regex-converter caveat for the pathological-nesting fallback. The gfm plugin ships no types; `src/turndown-plugin-gfm.d.ts` declares the one imported export over `@types/turndown` (a devDependency).
 
 The dependency-weight question the proposal flagged resolves in favor of the swap: `@deepseek-ai/dsh-tool-web` is in the single-file-executable closure ([single-exe note](../architecture/2026-07-10-single-file-executable-sdk-runtime-distribution.md)), and the exe's asset globs would pack ~7.9 MB of the three packages as published — but ~6 MB of that is `@mixmark-io/domino`'s test corpus (`test/**`), with runtime `lib/` at ~550 KB against a ~174 MB artifact, under 0.5% either way.
 
@@ -33,5 +33,5 @@ The previously-missing keyless `web_fetch` snapshot ships with the change as the
 
 ## Testing
 
-- `packages/web/tool-web/tests/tool-web.spec.ts` covers the turndown conversion surface (entities, links, tables, nesting, script/style/noscript removal) through `renderBody`, and the raw-HTML fallback with a measured reliably-overflowing 20k-level nesting input; per-file coverage on the package src is 100%.
+- `packages/web/tool-web/tests/tool-web.spec.ts` covers the turndown conversion surface (entities, links, tables, nesting, script/style/noscript removal) through `renderBody`, the fast raw-HTML passthrough for 20k-level nesting, the depth scan's void/self-closing/unbalanced cases, the residual converter-throw fallback, and the whole-output cap at expanding, exact, and tiny budgets; per-file coverage on the package src is 100%.
 - The `web-fetch` acp-agent snapshot pins the assembled behavior keylessly end to end (real Loader composition, real HTTP fetch, real conversion).