ci: build and test on Windows for every PR; harden Pullfrog

[lippdev]

Summary

• CI (.github/workflows/ci.yml, windows-latest): runs the unit tests and builds the app on every pull request and push to main. Until now no PR had any check; release.yml only runs on tags and manual dispatch.
• Pullfrog: sets up .NET 8 so the agent can run dotnet test on Linux, push: restricted (feature branches only; no pushes to main, no tag pushes, since a v* tag publishes a release), timeout: 30m. A commented model: line shows how to switch to Claude once an Anthropic key exists.
• AGENTS.md: tells agents the app only builds on Windows (CI does it), which tests they can run, and the repo conventions (three string catalogs, bilingual changelog and docs, no tags).

Validation

• This PR is the first run of the new CI.

🤖 Generated with Claude Code

[pullfrog]

• Check repository guidance and durable PR context
• Checkout PR and read the complete authoritative diff
• Trace changed CI and Pullfrog behavior and run targeted checks
• Update PR summary and submit exactly one review

  ^{｜ View workflow run ｜ via Pullfrog ｜ Using GPT Luna ｜ 𝕏}