Initial implementation of demon CLI tool

Implement complete daemon process management CLI with the following features:
- demon run: spawn background processes with stdout/stderr redirection
- demon stop: graceful process termination with SIGTERM/SIGKILL timeout
- demon tail: real-time file watching and log tailing
- demon cat: display log file contents
- demon list: show all managed processes with status
- demon status: detailed process information
- demon clean: remove orphaned files from dead processes

Technical implementation:
- Uses clap for CLI with enum-based subcommands
- Structured logging with tracing crate
- File watching with notify crate for efficient tailing
- Process management with proper signal handling
- Creates .pid, .stdout, .stderr files in working directory
- Comprehensive error handling and edge case coverage

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

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+# Demon CLI Implementation Plan
+
+## Project Overview
+A CLI tool named `demon` for spawning and managing background processes with stdout/stderr redirection.
+
+## Requirements Summary
+- **Files created**: `<id>.pid`, `<id>.stdout`, `<id>.stderr` in working directory
+- **Platform**: Linux only (maybe macOS later)
+- **File location**: Working directory (add to .gitignore)
+- **Signal handling**: SIGTERM then SIGKILL with configurable timeout
+- **Logging**: Tool logs to stderr, process output to stdout
+- **Concurrency**: Single process tail for now
+
+## CLI Structure
+```
+demon run --id <identifier> <command...>
+demon stop --id <id> [--timeout <seconds>]
+demon tail [--stdout] [--stderr] --id <id>
+demon cat [--stdout] [--stderr] --id <id>
+demon list
+demon status --id <id>
+demon clean
+```
+
+## Implementation Progress
+
+### ✅ Phase 1: Project Setup
+- [x] Add dependencies: clap, tracing, tracing-subscriber, notify
+- [x] Create CLI structure with Commands enum and Args structs
+
+### 🔄 Phase 2: Core Process Management
+- [ ] **CURRENT**: Implement `demon run`
+  - Check if process already running via PID file
+  - Spawn process with stdout/stderr redirection to files
+  - Write PID to `.pid` file
+  - Truncate log files when starting new process
+  - Detach process so parent can exit
+- [ ] Implement `demon stop`
+  - Read PID from file
+  - Send SIGTERM first
+  - Wait for timeout, then send SIGKILL
+  - Clean up PID file
+  - Handle already-dead processes gracefully
+
+### 📋 Phase 3: File Operations
+- [ ] Implement `demon cat`
+  - Read and display `.stdout` and/or `.stderr` files
+  - Handle file selection flags properly
+  - Error handling for missing files
+- [ ] Implement `demon tail`
+  - Use `notify` crate for file watching
+  - Support both stdout and stderr simultaneously
+  - Handle file rotation/truncation
+  - Clean shutdown on Ctrl+C
+
+### 📋 Phase 4: Additional Commands
+- [ ] Implement `demon list`
+  - Scan working directory for `.pid` files
+  - Check which processes are actually running
+  - Display process info
+- [ ] Implement `demon status`
+  - Check if specific process is running
+  - Display process info
+- [ ] Implement `demon clean`
+  - Find orphaned files (PID exists but process dead)
+  - Remove orphaned `.pid`, `.stdout`, `.stderr` files
+
+### 📋 Phase 5: Error Handling & Polish
+- [ ] Robust error handling throughout
+- [ ] Proper cleanup on failures
+- [ ] Input validation
+- [ ] Help text and documentation
+
+## Technical Implementation Details
+
+### Process Spawning (demon run)
+```rust
+// 1. Check if <id>.pid exists and process is running
+// 2. Truncate/create <id>.stdout and <id>.stderr files
+// 3. Spawn process with:
+//    - stdout redirected to <id>.stdout
+//    - stderr redirected to <id>.stderr
+//    - stdin redirected to /dev/null
+// 4. Write PID to <id>.pid file
+// 5. Don't call .wait() - let process run detached
+```
+
+### Process Stopping (demon stop)
+```rust
+// 1. Read PID from <id>.pid file
+// 2. Send SIGTERM to process
+// 3. Wait for timeout (default 10s)
+// 4. If still running, send SIGKILL
+// 5. Remove <id>.pid file
+// 6. Handle process already dead gracefully
+```
+
+### File Tailing (demon tail)
+```rust
+// 1. Use notify crate to watch file changes
+// 2. When files change, read new content and print
+// 3. Handle both stdout and stderr based on flags
+// 4. Default: show both if neither flag specified
+// 5. Graceful shutdown on Ctrl+C
+```
+
+### File Listing (demon list)
+```rust
+// 1. Glob for *.pid files in current directory
+// 2. For each PID file, check if process is running
+// 3. Display: ID, PID, Status, Command (if available)
+```
+
+## Dependencies Used
+- `clap` (derive feature) - CLI argument parsing
+- `tracing` + `tracing-subscriber` - Structured logging
+- `notify` - File system notifications for tail
+- Standard library for process management
+
+## File Naming Convention
+- PID file: `<id>.pid`
+- Stdout log: `<id>.stdout`  
+- Stderr log: `<id>.stderr`
+
+## Error Handling Strategy
+- Use `Result<(), Box<dyn std::error::Error>>` for main functions
+- Log errors using `tracing::error!`
+- Exit with code 1 on errors
+- Provide descriptive error messages
+
+## Testing Strategy
+- Manual testing with simple commands (sleep, echo, etc.)
+- Test edge cases: process crashes, missing files, etc.
+- Test signal handling and cleanup
+
+## Current Status
+- ✅ All core functionality implemented and tested
+- ✅ CLI structure with proper subcommands and arguments
+- ✅ Process spawning and management working correctly
+- ✅ File watching and real-time tailing functional
+- ✅ Error handling and edge cases covered
+- ✅ Clean up functionality for orphaned files
+
+## Implementation Complete!
+
+All planned features have been successfully implemented:
+
+1. **`demon run`** - ✅ Spawns background processes with file redirection
+2. **`demon stop`** - ✅ Graceful termination with SIGTERM/SIGKILL timeout
+3. **`demon tail`** - ✅ Real-time file watching with notify crate
+4. **`demon cat`** - ✅ Display log file contents
+5. **`demon list`** - ✅ Show all managed processes with status
+6. **`demon status`** - ✅ Detailed status of specific process
+7. **`demon clean`** - ✅ Remove orphaned files from dead processes
+
+## Testing Summary
+
+All commands have been tested and work correctly:
+- Process spawning and detachment
+- Signal handling (SIGTERM → SIGKILL)
+- File redirection (stdout/stderr)
+- Duplicate process detection
+- File watching and real-time updates
+- Orphan cleanup
+- Error handling for edge cases
+
+## Final Architecture
+
+The implementation follows the planned modular structure:
+- **CLI Interface**: Uses clap with enum-based subcommands ✅
+- **Process Manager**: Handles spawning, tracking, and termination ✅
+- **File Operations**: Manages PID files and log redirection ✅
+- **Output Display**: Implements both cat and tail functionality ✅
+
+The tool is ready for production use!
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