feat(docs): support all GitHub documentation languages

[matcra587]

Description

Documentation retrieval previously accepted English pages only. Add language to list_docs and search_docs, and infer the language for get_doc from its slug or URL. All nine current GitHub Docs languages are supported; English remains the default.

Missing translations return an explicit error with a separate English call when a matching page is known. Catalogues, refresh failures and fallback search are isolated by language while page and disk cache budgets remain shared. Non-English listings expose slugs without substituting English titles. Unicode snippets and heading retrieval preserve source bytes through pagination.

Review Focus

• Start with internal/docs/languages.go for language selection, cache keys and English alternatives, then the locale guards in client.go and apisearch.go.
• Review cursor validation and source-byte preservation together. Their tests cover large selections, Unicode, CRLF and restart behaviour.
• Tests account for most of this change. There are no dependency or generated-file changes; the implementation and its regression tests remain in one feature PR.

Test Procedure

• mise run check: lint, formatting, module checks, pinned vulnerability checker and shuffled race tests passed.
• mise run test:integration: live stdio MCP exchanges against docs.github.com passed in all nine languages. Reconstructed long pages matched independently fetched upstream bytes; non-English bodies differed from English.
• A separate live stdio traversal exhausted every catalogue and each actions filter without duplicate slugs, fetched two native-query search results per language, followed every page and section continuation, selected localized headings and replayed page/catalogue cursors after process restart with disk-cache evidence.
• Heading fuzzing passed 923,153 executions over 30 seconds. Controlled failures covered missing translations without English body fetches, foreign-language redirects and payloads, outages, recovery, cancellation, stale caches and cursor tampering.

Live results for the reviewed implementation:

Language	Catalogue entries	Workflow page bytes	Live result
English (en)	3,209	225,920	Pass
Spanish (es)	3,201	257,603	Pass
Japanese (ja)	3,201	233,671	Pass
Brazilian Portuguese (pt)	3,201	255,152	Pass
Simplified Chinese (zh)	3,201	206,349	Pass
Russian (ru)	3,201	341,219	Pass
French (fr)	3,201	270,995	Pass
Korean (ko)	3,201	239,899	Pass
German (de)	3,201	287,565	Pass

Each language also returned 245 entries under the actions filter. Article bodies were sampled; this does not establish correctness for every translation. Missing-translation alternatives and induced outages were tested against controlled origins.

Pre-flight Checklist

• Changes are limited to multilingual retrieval and its regression coverage
• Local checks and live tests pass
• README and tool descriptions updated

Additional Notes

Cursor format advances to version 2 because preserving source line endings changes selection offsets. Version 1 continuations return a restart error; initial tool calls and existing English cache entries remain supported.

GitHub can omit Content-Language. The server rejects declared mismatches and cross-language redirects but relies on upstream routing when no language is declared; it does not classify prose language. Draft pending maintainer validation.