feat(cli): shorten file paths in tool output display

Add three-level path shortening hierarchy for cleaner CLI output:
1. Project path -> <project_name>/... (when project loaded via /project)
2. Workspace path -> ./... (relative to current working directory)
3. Home path -> ~/... (fallback for paths under home directory)

Changes:
- Add shorten_path() and shorten_paths_in_command() functions in display.rs
- Add project_path/project_name fields to ConsoleUiWriter
- Add set_workspace_path(), set_project_path(), clear_project() to UiWriter trait
- Add ui_writer() getter to Agent struct
- Wire up project path setting in /project and /unproject commands
- Set workspace path when creating agents in all CLI modes

Before: ● read_file | /Users/dhanji/icloud/butler/projects/appa_estate/status.md
After:  ● read_file | appa_estate/status.md (with project loaded)
        ● read_file | ./src/main.rs (workspace-relative)
        ● read_file | ~/Documents/file.txt (home-relative)

crates/g3-core/src/lib.rs

@@ -1055,6 +1055,11 @@ impl<W: UiWriter> Agent<W> {
         &self.context_window
     }
 
+    /// Get a reference to the UI writer.
+    pub fn ui_writer(&self) -> &W {
+        &self.ui_writer
+    }
+
     /// Add a message directly to the context window.
     /// Used for injecting discovery messages before the first LLM turn.
     pub fn add_message_to_context(&mut self, message: Message) {