chore(release): prepare 0.12.0

[srothgan]

Summary

• Prepare the 0.12.0 release by bumping the crate version and lockfile metadata.
• Add concise 0.12.0 changelog notes for the inline rendering runtime, resolved keymap, README updates, dependency refreshes, and the rustls-webpki advisory fix.
• Move the Agent SDK billing warning below README usage and keep the outdated Architecture section removed.

Why

This branch packages the rendering and keymap work already merged to main into the 0.12.0 release, while also resolving the open rustls-webpki advisory before publishing.

Closes #153

Validation

• Automated:
  • cargo audit
  • cargo check --offline
  • cargo fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test
  • git diff --check
• Manual:
  • Verified rustls-webpki resolves to 0.103.13.
  • Verified the README warning is below Usage and no README Architecture section remains.
• Screenshot/video (if UI changed): N/A

Notes

• Breaking changes: N/A
• Docs updated: Yes