[codex] Add repository guidelines

[jjoonleo]

Summary

Adds AGENTS.md as a concise contributor guide for this Flutter repository.

Details

• Documents project structure across lib/, test/, assets, platform folders, and docs.
• Lists common Flutter development, build, generation, and test commands.
• Captures Dart style, testing, commit, PR, and agent-specific guidance.

Validation

• Reviewed the generated Markdown content.
• Confirmed the document is 385 words, within the requested 200-400 word range.
• Did not run Flutter tests because this is a documentation-only change.

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