chore: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Prerequisites
+
+- rust 1.85+ (edition 2024)
+- sqlite development libraries
+
+install the rust toolchain via [rustup](https://rustup.rs/) and make sure
+sqlite headers are available for your platform.
+
+## Development
+
+```sh
+cargo build
+cargo test
+cargo clippy
+cargo fmt
+```
+
+run all checks before submitting changes:
+
+```sh
+cargo fmt && cargo clippy && cargo test && cargo build
+```
+
+### Database
+
+data is stored in a sqlite file at `$XDG_DATA_HOME/todo/todos.db` (override
+with `TODO_DB`). migrations in `migrations/` are applied automatically at
+startup.
+
+## Project layout
+
+```
+src/
+  main.rs       - entry point
+  cli.rs        - clap argument definitions
+  commands.rs   - command handlers
+  db.rs         - database access layer
+  model.rs      - data types
+  display.rs    - output formatting
+  mcp.rs        - mcp server
+tests/          - integration tests
+migrations/     - sqlite migrations
+build.rs        - shell completion generation
+```
+
+## Submitting changes
+
+1. create a feature branch from `main`
+2. keep commits small and focused
+3. ensure `cargo fmt`, `cargo clippy`, and `cargo test` all pass
+4. open a merge request with a clear description of what changed and why