docs: add Brazilian Portuguese and Russian READMEs - #770

Description

Brazilian Portuguese and Russian readers currently have no localized entry point. Add two abridged READMEs covering the project, supported engines, installation, quick start, security, AI data flow, and contribution requirements. Both translations share the language-switcher and drift-guard update in this PR.

Type of Change

• Documentation update
• Test addition or update

Related Issue

Closes #594
Closes #610

Claims: Portuguese, Russian.

Changes Made

• Add README_pt.md (251 lines) and README_ru.md (254 lines), following the existing abridged scope. Preserve the 16 canonical engine names and quote install-table commands verbatim.
• Keep deb/rpm, AppImage, Snap, standalone tarball, and npx/npm visible, including the distinction between the server and desktop packages. Explain the agent's supported engines and what the configured model endpoint receives.
• Update all six language switchers, including the Spanish README already on main, and register both new files in LOCALIZED.
• Add four CLI regression cases: a missing engine and a changed install command in each new language must fail the guard. All four failed before registration and pass afterward.

Testing

• Tested locally
• Added regression tests
• All existing tests pass

Local validation on a767100f700c10cf44a4de0aaca02b4c00b8f31c:

• bun run test:unit --isolate --pass-with-no-tests -t 'parseTables|table lookup|extraction|checkReadmes|readme-check CLI': 45 passed, including all 31 drift-guard tests. The name filter also selected 14 existing tests.
• bun run readme:check: both new translations match the canonical engine set and install commands.
• bun run format, lint, typecheck, knip, chart:check, channels:showcase:check, security:check, and build: passed.
• Verified all six switchers and all 64 relative file links in the new READMEs; reviewed both translations against the English README and the distribution/agent documentation.

The complete local bun run test / coverage run was not repeated: this Windows environment lacks the Helm/chart test prerequisites and has known unrelated SQLite file-lock limitations. Full-suite, coverage, E2E and engine checks are left to the required CI workflows. No installation command in the documentation was executed as part of translation validation.

Test Environment

• Base: 2f4ae61216956824410ba2f16bae3f697348afe3
• OS: Windows x64
• Node.js: 24.18.1; Bun: 1.4.2
• Browser / database: not applicable to this documentation change

Checklist

• My code follows the project's code style guidelines
• I have performed a self-review of my code
• I have updated the documentation accordingly
• I have added tests that prove my fix is effective or that my feature works
• New and existing targeted unit tests pass locally with my changes
• The required CI test job passes the 100% line-coverage gate (bun run test:coverage and bun run coverage:check)
• Provider triad: not applicable; no provider files changed.
• Dependencies: both issues are included together so the language-switcher links resolve within this PR.

Additional Notes

Prepared with Codex, including AI-assisted translation and test authoring. Technical claims, terminology, commands, and links were reviewed and revised against the source documents; no independent native-speaker review is claimed. Language-quality feedback is welcome. The drift guard verifies technical invariants, not linguistic quality.