docs: add API deprecation policy foundation (1%)

[coderolisa]

• Created initial API_DEPRECATION_POLICY.md document
• Defined deprecation response header structure
• Specified X-API-Deprecation header format with version, sunset date, and migration link
• Foundation for complete deprecation lifecycle management

Overview

Related Issue

Closes #993

Changes

Verification

# e.g.
cd dashboard && npm test
cd listener && npm run typecheck && npm test
cd contract/contracts/hello-world && cargo test

How to Test

Checklist

• Branch is up to date with main
• Tests added/updated and all pass locally
• cargo fmt --all run (if Rust changes)
• npm run lint passes (if TypeScript changes)
• Documentation updated if behavior changed

[drips-wave]

@coderolisa Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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