docs: sync provider registry, test inventory, and Gemini page with the tree

[iamtoruk]

Maintainer docs pass, written from the code on main.

• docs/providers/README.md: add the missing Codebuff, Mux, Open Design and Zed rows; Gemini and Antigravity now point at their real test files instead of none.
• docs/architecture.md: eager provider list gains cline-cli, kimicode, openclaude, quickdesk; test inventory and lock-suite count corrected (four cache-refresh-lock suites).
• CONTRIBUTING.md: the npm test row no longer hard-codes a case count that rots on every PR, and the full-suite gate now says npm test (plus cd app and npm test for app changes, swift test under mac/) instead of the unsupported whole-repo npx vitest run that sweeps app/ and fails on jsdom.
• docs/providers/gemini.md: one call per assistant message with usage (not per session), JSON.parse then JSONL fallback (not a first-character sniff), dedup key per session and message, stale line citations dropped.
• New pages docs/providers/codebuff.md and docs/providers/open-design.md derived from their parsers.

Verification: npm test in a fresh worktree, 260 passed, 2 skipped, 0 failed; every cited path checked with ls; no em-dashes added.