[codex] Apply release branch deployment strategy

[jjoonleo]

Summary

• Documents trunk-based development with release branches, hotfix branches, and v* production tags.
• Aligns CI triggers so main pushes deploy staging/test builds while release/hotfix branches and production tags run release verification.
• Adds an ENV Dart define with a dev default and updates build docs for dev, staging, and prod.

Validation

• flutter analyze
• flutter test
• GitHub Actions YAML parsed with Ruby YAML loader
• git diff --check

[github-actions]

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[github-actions]

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