Sync provider and test documentation

[dajiaohuang]

Summary

• update contributor and architecture test inventories to the current 265-file tree and four serial lock suites
• make the provider index match the 41-provider registry, including current Gemini and Antigravity tests
• add the missing Codebuff and Open Design provider maintenance pages

Fixes #1206.

Testing

• I have tested this locally against real data (not just unit tests)
• npm test passes
• npm run build succeeds

Documentation-focused validation performed:

• npx tsc --noEmit
• 77 Markdown files / 64 local Markdown targets checked, with 0 broken targets
• provider registry/index consistency check: 41 providers, no undocumented registered provider (OMP intentionally shares pi.ts)
• tracked test inventory check: 265 total, 261 parallel, 4 lock suites
• npx vitest run tests/providers/codebuff.test.ts tests/providers/gemini.test.ts (26 passed)
• Electron app typecheck, Electron compile, and renderer build
• dashboard typecheck and production build
• Windows menubar TypeScript/Vite build
• git diff --check

The full root and Electron suites are green in the upstream CI for the exact base commit. On this native Windows host, the root suite and four Open Design cases hit existing POSIX-path assumptions, while Electron process-spawn cases hit native Windows EFTYPE; no runtime or test code is changed here.

For new providers only:

• I installed the tool and generated real sessions by using it
• npm run dev -- today shows correct costs and session counts for this provider
• npm run dev -- models --provider <name> shows correct model names and pricing
• Screenshot or terminal output attached below proving it works with real data

Not applicable: this PR documents existing providers and does not add or change a provider implementation.