docs: refresh release highlights for v0.3.18

[Copilot]

This updates the release summary doc to reflect v0.3.18 instead of the stale v0.3.6 content. The new summary aligns the documented highlights with the release scope: WASM guard hardening, DIFC refactors, reliability fixes, and expanded test coverage.

• Version summary replacement

  • Replaced the v0.3.6 section with a v0.3.18-focused narrative and updated thematic sections:
    • ✨ What’s New
    • 🐛 Bug Fixes & Improvements
    • 🔬 Testing & Reliability

• Highlight content alignment

  • Added the v0.3.18 items for:
    • WASM guard robustness (call_backend size-hint handling, larger buffers, cache lock improvements, fallback coverage)
    • DIFC flags module refactor
    • Config map expansion and shared applyFlagOrEnv helper
    • Rust guard and collaborator-permission test coverage improvements
    • GraphQL rewrite debug logging note

• Release artifact metadata

  • Updated Docker pull example tag to v0.3.18
  • Updated release link to releases/tag/v0.3.18

## v0.3.18

### ✨ What's New
- **WASM guard robustness** ...
- **DIFC flags module** ...

docker pull ghcr.io/github/gh-aw-mcpg:v0.3.18

[Copilot]

Pull request overview

Updates the repository’s release highlight summary to reflect the v0.3.18 release, replacing the previously stale v0.3.6 content in the user-facing release notes documentation.

Changes:

• Replaces the v0.3.6 narrative with a v0.3.18-focused summary and reorganized highlight sections.
• Updates listed highlights to match v0.3.18 scope (WASM guard hardening, DIFC refactors, config/flag-env helper work, test/diagnostics improvements).
• Updates the Docker pull example and the GitHub release link to v0.3.18.

Show a summary per file

File	Description
docs/RELEASE_NOTES.md	Refreshes the release highlights content and links to reference v0.3.18.

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