docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Prerequisites
+
+- [Rust](https://rustup.rs) stable (includes `cargo`, `rustfmt`, `clippy`)
+- [`jq`](https://jqlang.github.io/jq/) — required at runtime by `ack.sh`
+
+## Build
+
+```sh
+cargo build --manifest-path core/Cargo.toml
+```
+
+Release binary:
+
+```sh
+cargo build --manifest-path core/Cargo.toml --release
+# binary: core/target/release/steplock
+```
+
+## Test
+
+```sh
+cargo test --manifest-path core/Cargo.toml
+```
+
+## Code quality
+
+CI enforces all three — run them before pushing:
+
+```sh
+cargo fmt  --manifest-path core/Cargo.toml
+cargo clippy --manifest-path core/Cargo.toml --all-targets -- -D warnings
+RUSTDOCFLAGS="-D warnings" cargo doc --manifest-path core/Cargo.toml --no-deps
+```
+
+## Project layout
+
+```
+core/          Rust crate (library + binary)
+scripts/       Shell scripts embedded in the binary (ack.sh)
+schemas/       JSON Schema files for config.toml and state.json
+examples/      Working reference configurations
+```
+
+## Making changes
+
+- **New checklist logic** — add to `core/src/run.rs`; unit tests live in the same file
+- **CEL evaluation** — `core/src/cel_eval.rs`
+- **Mermaid parsing** — `core/src/flow.rs`
+- **Shell scripts** — edit `scripts/ack.sh`; the binary embeds it via `include_str!` so a rebuild picks up your changes
+- **Schema** — update `schemas/checklist-config.schema.json` and the corresponding Rust struct in `core/src/config.rs`
+
+## Submitting a pull request
+
+1. Fork and create a branch off `main`
+2. Make your change with tests
+3. Ensure `cargo test`, `cargo fmt --check`, and `cargo clippy` all pass
+4. Open a draft PR — the CI suite runs automatically
+5. Mark as ready when CI is green