feat: improve Codeck onboarding and automation

[isdou]

What changed

This PR addresses the four growth blockers identified for Codeck:

• sharpens the product positioning around Codex-first local context handoff;
• moves the npm package to the scoped @isdou/codeck name while keeping the codeck binary;
• replaces the clone/build/link instructions with a five-minute Chinese and English quick start;
• adds --json output for auto, route, ask, delegate, and compare, including structured child runs for compare;
• adds a contributor guide and a reproducible bug-report template.

Verification

• npm ci
• npm test (10/10)
• npm run test:gemini-image
• npm pack --dry-run
• installed the packed tarball into a clean temporary prefix and verified init, ask --json mock, and doctor

The scoped npm package is prepared but not published in this PR; publishing requires npm authentication and should happen after review/merge.