Add AGENTS.md with coding agent instructions

AGENTS.md

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+# AGENTS.md
+
+Instructions for AI coding agents working on pullpito.
+
+## Development Environment
+
+- Rust toolchain via `rustup` (see README.md for installation)
+- Project uses Rust 2024 edition
+
+## Build and Test Commands
+
+```bash
+# Run tests
+cargo test
+
+# Format code
+cargo fmt
+
+# Run linter
+cargo clippy
+
+# Build and run
+cargo run -- --repository <org/repo>
+
+# Build release binary
+cargo build --release
+```
+
+## Project Structure
+
+- `src/main.rs` - Entry point
+- `src/lib.rs` - Main library with CLI parsing and statistics logic
+- `src/github_events/mod.rs` - GitHub API interaction module
+- `test/` - JSON fixtures for tests
+
+## Key Implementation Details
+
+### GitHub API Integration
+- Fetches events from GitHub REST API (`/repos/{repo}/events`)
+- Supports pagination via Link headers
+- Handles authentication via optional token
+- Uses blocking HTTP client (reqwest)
+
+### Event Processing
+- Filters for: `PullRequestEvent`, `PullRequestReviewCommentEvent`, `IssueCommentEvent`
+- Groups events by actor/author
+- Tracks actions: `opened`, `created`, `closed`
+
+### Concurrency
+- Multi-repository queries run in parallel using threads and channels
+- Each repository is processed in its own thread
+
+## Coding Conventions
+
+- Use `#[allow(non_camel_case_types)]` for enum variants matching GitHub API response format
+- Deserialize with custom handlers for enum fields to handle unknown variants gracefully
+- Prefer `unwrap()` for expected operations (API failures are acceptable panics per doc comments)
+- Use `lazy_static!` for compiled regex patterns
+- Format strings with `{var:?}` for debug output, `{var}` for display
+- Keep test fixtures in `test/` directory with descriptive names
+
+## Testing Practices
+
+- Unit tests in same file as implementation using `#[cfg(test)] mod tests`
+- Test fixtures stored as JSON files in `test/` directory
+- Use `include_str!()` macro to load test fixtures
+- Test both happy paths and edge cases (unknown enums, pagination, etc.)
+
+## Dependencies
+
+Key dependencies to be aware of:
+- `structopt` - CLI argument parsing
+- `serde`/`serde_json` - JSON serialization
+- `reqwest` - HTTP client (blocking mode)
+- `chrono` - DateTime handling
+- `log`/`env_logger` - Logging infrastructure
+
+## Logging
+
+- Use `log` crate macros: `info!`, `debug!`, `trace!`
+- Controlled via `RUST_LOG` environment variable
+- `trace` level may contain sensitive data (tokens)