feat: add background_process tool for launching long-running processes

Adds a new tool that allows launching processes (like game servers) in the
background while g3 continues to operate. The process runs independently
with stdout/stderr captured to a log file.

Features:
- Named process tracking for easy reference
- Automatic log capture to logs/background_processes/
- Returns PID and log file path for use with shell tool
- Automatic cleanup on agent shutdown via Drop trait

Usage: Use shell tool to interact with the process:
- Read logs: tail -100 <logfile>
- Check status: ps -p <pid>
- Stop process: kill <pid>

Files:
- New: crates/g3-core/src/background_process.rs
- New: crates/g3-core/tests/background_process_demo_test.rs
- Modified: crates/g3-core/src/lib.rs (tool definition + handler)
- Modified: crates/g3-core/src/prompts.rs (documentation)

crates/g3-core/src/prompts.rs

@@ -248,6 +248,12 @@ Short description for providers without native calling specs:
   - Format: {\"tool\": \"shell\", \"args\": {\"command\": \"your_command_here\"}
   - Example: {\"tool\": \"shell\", \"args\": {\"command\": \"ls ~/Downloads\"}
 
+- **background_process**: Launch a long-running process in the background (e.g., game servers, dev servers)
+  - Format: {\"tool\": \"background_process\", \"args\": {\"name\": \"unique_name\", \"command\": \"your_command\"}}
+  - Example: {\"tool\": \"background_process\", \"args\": {\"name\": \"game_server\", \"command\": \"./run.sh\"}}
+  - Returns PID and log file path. Use shell tool to read logs (`tail -100 <logfile>`), check status (`ps -p <pid>`), or stop (`kill <pid>`)
+  - Note: Process runs independently; logs are captured to a file for later inspection
+
 - **read_file**: Read the contents of a file (supports partial reads via start/end)
   - Format: {\"tool\": \"read_file\", \"args\": {\"file_path\": \"path/to/file\", \"start\": 0, \"end\": 100}
   - Example: {\"tool\": \"read_file\", \"args\": {\"file_path\": \"src/main.rs\"}