Ensure all tests run on all platforms without skipping

- Remove test skipping based on target availability
- Install cross-compilation toolchains for all platforms in CI:
  - Ubuntu: musl-tools
  - macOS: musl-cross with proper cargo config
  - Windows: rust-lld linker
- Update builder to use platform-appropriate linkers
- Make full build/run workflow test mandatory in CI
- Add Docker setup and local registry for integration tests
- Set RUSTFLAGS with --cfg ci to enable CI-specific test behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -24,6 +24,7 @@ tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 dirs = "5.0"
 toml = "0.8"
+which = "6.0"
 
 [dev-dependencies]
 tempfile = "3.9"