Add 2025 reflections post and polish contribution workflow docs

[beingzy]

Summary

This PR adds the year-end post “Codatta 2025: Reflections and 2026 Forge” and improves the contributor documentation for writing posts and using Git/GitHub.

Changes

• Added new post: _posts/2025-12-31-codatta-2025-reflections-and-2026-forge.markdown
  • Author: yi_zhang
  • Category: Ecosystem & Community
  • Tags: community
  • Content: 2025 recap, macro context, and 2026 “Forge” roadmap
• Kept existing README structure while clarifying:
  • Overall contribution flow (clone → branch → write → validate → PR → deploy)
  • Git & GitHub workflow (branch naming, commits, PR review, merge/deploy expectations)

Motivation

• Capture a clear, narrative summary of what Codatta achieved in 2025 and what “Forge” means for 2026.
• Make it easier for contributors to onboard and follow a consistent Git workflow when adding or editing posts.

Testing

• bundle exec jekyll serve (locally) and verified the new post renders correctly
• Scanned README for formatting/links and rendered it in a Markdown preview

Checklist

• Content reviewed for accuracy (dates, links, and partner mentions)
• Copy/style reviewed (tone, typos, headings)
• Reviewer(s) assigned

[modernchina123]

Looks Good!