📘 Add WARP.md: dev commands, architecture, config, testing, and notes

WARP.md

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+# WARP.md
+
+This file provides guidance to WARP (warp.dev) when working with code in this repository.
+
+## Core Development Commands
+
+### Build and Test
+- Build: `cargo build`
+- Test: `cargo test`
+- Format: `cargo fmt --all`
+- Lint: `cargo clippy --all-targets --all-features -- -D warnings`
+- CI test runner: `cargo nextest run --profile ci`
+
+### CLI Development
+- Install CLI locally: `cargo install --path .`
+- Run CLI without installing: `cargo run --bin cull-gmail -- --help`
+- Test a single test by pattern: `cargo test test_rules_new_creates_default_rules`
+
+### Special Tests
+- Run ignored Gmail API integration test: `cargo test --test gmail_message_list_integration -- --ignored`
+  - Requires valid OAuth2 credentials configured
+  - Only use with test Gmail account
+
+## High-level Architecture
+
+### Crates
+- **Library**: `cull_gmail` - Core Gmail management functionality
+- **Binary**: `cull-gmail` - CLI application
+
+### Core Components
+- **GmailClient**: Async Gmail API client with OAuth2 authentication, refresh token persistence, and Gmail API request helpers
+- **Rules/EolRule**: Rule-based message lifecycle engine that loads from TOML configuration and evaluates messages to yield actions (trash/delete)
+- **ClientConfig**: Configuration loader that merges TOML files with environment variables using `APP_` prefix
+- **MessageList**: Abstraction over sets of Gmail message IDs with support for batch list, trash, and delete operations
+- **CLI**: Command-line interface with subcommands for `labels`, `messages`, `rules`, and `token`; implements dry-run-first behavior for safety
+
+### CLI Subcommands
+- `labels` - List and inspect available Gmail labels
+- `messages` - Query, filter, and perform batch operations on Gmail messages
+- `rules` - Configure and execute automated message retention rules
+- `token` - Export/import OAuth2 tokens for ephemeral environments
+
+### Runtime
+- **Async/await**: All network operations use Tokio async runtime
+- **OAuth2**: Uses installed application flow with token caching to `~/.cull-gmail/gmail1/`
+- **Safety**: Dry-run mode by default; destructive actions require explicit execution flags
+
+## Configuration
+
+### Directory Structure
+- **Config directory**: `~/.cull-gmail/`
+- **Main config**: `cull-gmail.toml` 
+- **Rules config**: `rules.toml`
+- **OAuth2 tokens**: `gmail1/` subdirectory
+
+### Configuration Sources (in precedence order)
+1. Environment variables with `APP_` prefix
+2. TOML configuration files
+3. Built-in defaults
+
+### OAuth2 Setup Requirements
+1. Google Cloud Console project with Gmail API enabled
+2. OAuth2 desktop application credentials
+3. Download credential JSON to config directory
+4. First run triggers browser authentication flow
+
+## Testing
+
+### Test Organization
+- **Unit tests**: Alongside modules in `src/`
+- **Integration tests**: In `tests/` directory
+- **CI runner**: Uses nextest with `ci` profile from `.config/nextest.toml`
+
+### Test Categories
+- Standard tests: `cargo test` or `cargo nextest run --profile ci`
+- Gmail API integration test (ignored by default): Requires valid credentials and network access
+
+### Important Notes
+- **MSRV**: Rust 1.88+ (edition 2024)
+- **Required runtime**: Tokio for all async operations
+- **Gmail API**: Requires valid OAuth2 credentials for real testing
+- **Logging**: Use `RUST_LOG=cull_gmail=debug` for detailed operation logs
+- **Safety**: All operations default to dry-run mode; use explicit execution flags for actual changes