V12: Link error docs to lousho.com

[LinuxDevil]

Fixes audit finding DX-7 (Vercel competitor audit, 2026-10-19).

What was wrong

Every SDKError docs link pointed at raw markdown on GitHub blob/main, which may not match the installed version, instead of the docs site.

What changed

• ERROR_DOCS_URL is now https://lousho.com/errors. Verified the site serves docs/errors.md there and renders per-code anchors with underscores kept (e.g. #lousho_registry_unreachable), so the anchor format is unchanged.
• Updated the sample line in docs/errors.md, regenerated llms.txt/llms-full.txt and the API report.
• Updated tests that pinned the old URL.

Tests

• New errorCodes test: asserts https://lousho.com/errors#lousho_registry_unreachable and that every code has a heading in docs/errors.md.
• tsc, typecheck:tests, lint, api:check pass. Full test:coverage: only environment/load flakes (timing tests that pass alone; create-lousho-agent needed its dist built). fallow could not run because vitest does not write coverage when any test fails under load.

Changelog: SDKError docs links now point to https://lousho.com/errors# instead of GitHub blob/main.

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