chore: add Cargo.toml OSS metadata and CONTRIBUTING.md

Cargo.toml had no license/repository/readme/keywords/categories fields
— expected metadata for a public Rust project. Added CONTRIBUTING.md
(build from source, pre-PR checks, code layout) since the repo had no
onboarding doc for contributors beyond the CI gate added earlier this
round; linked from README.

Cargo.toml

@@ -4,6 +4,11 @@ version = "0.1.0"
 edition = "2021"
 description = "Multi-repo git workflow manager"
 authors = ["dualfroz"]
+license = "MIT"
+repository = "https://github.com/dualfroz/fxg"
+readme = "README.md"
+keywords = ["git", "tui", "cli", "workflow", "productivity"]
+categories = ["command-line-utilities", "development-tools"]
 build = "build.rs"
 
 [[bin]]