chore(release): bump version to 0.1.20

[Wangnov]

Summary

• bump Codex App Manager to 0.1.20 across package, Tauri, and Cargo metadata
• add docs/releases/v0.1.20.md for the Windows short MSIX URL fix

Verification

• jq empty package.json package-lock.json src-tauri/tauri.conf.json
• npm run build
• npm test
• cargo test --manifest-path src-tauri/Cargo.toml --workspace
• cargo clippy --manifest-path src-tauri/Cargo.toml --workspace --all-targets -- -D warnings
• codex review --base main

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