Improved README with architecture, setup, and contribution guidelines

[Shubham1905s]

This PR improves the project README by:

• Adding detailed architecture overview
• Documenting local/remote OpenCode integration
• Improving development setup instructions
• Clarifying plugin system usage
• Adding contribution and security sections

No code changes included — documentation only.

Happy to update if needed 🙂

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[benjaminshafii]

Automated message: thanks for the doc pass. A few accuracy issues need correction before merge:

• UI is SolidJS, not React.
• Default contribution base is dev (not main).
• The repo does not have .opencode/skill/prd-conventions/SKILL.md here; please reference the actual OpenWork PRD convention or remove that pointer.
• Double-check paths and terms to align with AGENTS.md/MOTIVATIONS-PHILOSOPHY.md (avoid adding new abstractions).

Once corrected, I’m happy to re-review.