chore: Dogfood Fabrik builds

Summary

• Adds a cargo_binary target for Cargo-backed Rust binaries.
• Adds a root //:fabrik target so the release binary can build Fabrik through Fabrik.
• Runs target/release/fabrik targets and target/release/fabrik run //:fabrik in CI.
• Documents the git-cliff release tasks and the dogfood commands in the README.

Notes

Release automation was already present through cliff.toml, mise/tasks/release/*, and .github/workflows/release.yml. This PR fills the README gap and adds the first dogfood path.

Validation

• mise exec -- cargo test --workspace
• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo build --release
• mise exec -- target/release/fabrik targets
• mise exec -- target/release/fabrik run //:fabrik
• mise exec -- target/release/fabrik run //:fabrik again, verifying a cache hit
• mise exec -- shellspec
• mise run release:detect

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