Agent Mode Enhancements

• Agent prompts are now embedded within the g3 binary
• README.md - Added new "Agent Mode" section documenting:
  • All 7 built-in agents with their focus areas
  • Usage examples (--list-agents, --agent <name>)
  • How to create custom workspace agents

Behavior
1. Workspace agents take priority - If agents/<name>.md exists in the workspace, it's used
2. Embedded fallback - If no workspace agent exists, the embedded version is used
3. Portability - g3 binary now works on any repo without needing the agents/ directory
4. Discoverability - g3 --list-agents shows all available agents and their source

crates/g3-cli/src/cli_args.rs

@@ -99,6 +99,10 @@ pub struct Cli {
     #[arg(long, value_name = "NAME", conflicts_with_all = ["autonomous", "auto", "chat", "planning"])]
     pub agent: Option<String>,
 
+    /// List all available agents (embedded and workspace)
+    #[arg(long)]
+    pub list_agents: bool,
+
     /// Skip session resumption and force a new session (for agent mode)
     #[arg(long)]
     pub new_session: bool,