docs: add CONTRIBUTING.md and refresh README header

[picsoung]

I was trying to contribute and had issues finding out how to run the project locally.
Inspired by the OpenClaw's CONTRIBUTING.md and the README, decided to improve the ones here.

Summary

• Add CONTRIBUTING.md with local Convex setup instructions — previously
  undocumented steps (env vars, GitHub OAuth, JWT keys, database seeding)
  were only discovered through trial and error
• Refresh the README header with the centered lobster logo, badges, and a
  quick links row linking to live sites, docs, and the new contributing guide
• Condense the README "Local dev" section and link to CONTRIBUTING.md for
  full details
• Convert plain-text docs/ references to clickable markdown links

Test plan

• Verify the logo renders correctly on GitHub (centered, 120px)
• Confirm all quick links row URLs resolve
• Confirm all docs/ links in the README point to existing files
• Read through CONTRIBUTING.md for accuracy (env values, commands, callback URL)
• Run bun run lint and bun run build to confirm no regressions

Questions

I could not find a dedicated ClawHub channel on Discord to reference in the doc.

AI disclosure

This PR was authored with Claude Code. All content was reviewed and
validated against the actual codebase (env var references, doc paths,
seed commands). The contributor (me 👋) understands and can maintain these files.

🤖 Generated with Claude Code

Greptile Summary

Adds a new CONTRIBUTING.md with detailed local development setup instructions (env vars, GitHub OAuth, JWT keys, database seeding, CLI testing) and refreshes the README.md header with a centered logo, badge row, and quick links. The README's "Local dev" section is condensed and now links to CONTRIBUTING.md for full details. Plain-text doc references are converted to clickable markdown links.

• All referenced file paths (docs/, packages/clawdhub/, public/clawd-logo.png, VISION.md) verified to exist
• Seed function names (devSeed:seedNixSkills, devSeedExtra:seedExtraSkillsInternal) match the exported Convex functions
• The #setup-help Discord channel referenced in CONTRIBUTING.md may not exist — the PR author noted they couldn't find a dedicated ClawHub Discord channel

Confidence Score: 4/5

• Documentation-only PR with no code changes — safe to merge after verifying the Discord channel reference.
• This is a docs-only PR (new CONTRIBUTING.md + README refresh) with no functional code changes. All referenced file paths, seed commands, and env vars were verified against the codebase. The one concern is a Discord channel name that may not exist, which could mislead new contributors but has no runtime impact.
• CONTRIBUTING.md line 5 references a #setup-help Discord channel that may not exist.

_{Last reviewed commit: b39114c}

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Context used:

• Context from dashboard - AGENTS.md (source)

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