Update documentation for proto-flutter-plugin

[google-labs-jules]

Updated project documentation to reflect current state.

• Created CONTRIBUTING.md linking to development guide.
• Updated README.md to link to CONTRIBUTING.md and remove outdated Windows limitation.
• Updated AGENTS.md to reference CONTRIBUTING.md and docs/.

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PR created automatically by Jules for task 8921233825625114345 started by @rpsec

[google-labs-jules]

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