Simplify --agent --chat startup: minimal output, no session resume

When running g3 --agent <name> --chat, the output is now minimal:
- Workspace path (-> ~/path)
- Status line (README/AGENTS.md/Memory)
- Context progress bar
- Prompt (g3>)

Skipped in this mode:
- Session resume prompts
- "agent mode | name (source)" header
- "g3 programming agent" welcome
- Provider info display
- Language guidance messages

Added from_agent_mode parameter to run_interactive() to control
whether verbose welcome and session resume are shown.

crates/g3-cli/src/lib.rs

@@ -218,6 +218,7 @@ async fn run_console_mode(
             combined_content,
             project.workspace(),
             cli.new_session,
+            false, // from_agent_mode
         )
         .await
     }