Add --project CLI flag for loading projects at startup

Adds a new --project <PATH> flag that loads project files (brief.md,
contacts.yaml, status.md) at startup, similar to the /project command
but WITHOUT auto-executing the project status prompt.

Changes:
- Add --project flag to cli_args.rs
- Add load_and_validate_project() helper in project.rs (shared by both
  --project flag and /project command)
- Modify run_interactive() to accept optional initial_project parameter
- Wire up --project in lib.rs to load project before interactive mode
- Refactor /project command to use shared helper (reduces duplication)
- Add 4 new tests for load_and_validate_project()

crates/g3-cli/src/agent_mode.rs

@@ -255,6 +255,7 @@ pub async fn run_agent_mode(
             &workspace_dir,
             new_session,
             Some(agent_name),  // agent name for prompt (e.g., "butler>")
+            None, // initial_project (not supported in agent mode yet)
         )
         .await;
     }