feat(phase5): implement mode consolidation and tool availability system

Implements Phase 5 from the roadmap with complete mode-based tool filtering:

- Add Mode enum (Chat/Code) with FromStr trait implementation
- Extend Config with ModeConfig for per-mode tool availability
- Update ToolRegistry to enforce mode-based filtering
- Add --code/-c CLI argument to start in code mode
- Implement TUI commands: :mode, :code, :chat, :tools
- Add operating mode indicator to status line (💬/💻 badges)
- Create comprehensive documentation in docs/phase5-mode-system.md

Default configuration:
- Chat mode: only web_search allowed
- Code mode: all tools allowed (wildcard *)

All code compiles cleanly with cargo clippy passing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/owlen-cli/src/main.rs

@@ -1,7 +1,8 @@
 //! OWLEN CLI - Chat TUI client
 
 use anyhow::Result;
-use owlen_core::{session::SessionController, storage::StorageManager};
+use clap::Parser;
+use owlen_core::{mode::Mode, session::SessionController, storage::StorageManager};
 use owlen_ollama::OllamaProvider;
 use owlen_tui::tui_controller::{TuiController, TuiRequest};
 use owlen_tui::{config, ui, AppState, ChatApp, Event, EventHandler, SessionEvent};
@@ -17,11 +18,22 @@ use crossterm::{
 };
 use ratatui::{prelude::CrosstermBackend, Terminal};
 
+/// Owlen - Terminal UI for LLM chat
+#[derive(Parser, Debug)]
+#[command(name = "owlen")]
+#[command(about = "Terminal UI for LLM chat with Ollama", long_about = None)]
+struct Args {
+    /// Start in code mode (enables all tools)
+    #[arg(long, short = 'c')]
+    code: bool,
+}
+
 #[tokio::main(flavor = "multi_thread")]
 async fn main() -> Result<()> {
-    // (imports completed above)
+    // Parse command-line arguments
+    let args = Args::parse();
+    let initial_mode = if args.code { Mode::Code } else { Mode::Chat };
 
-    // (main logic starts below)
     // Set auto-consent for TUI mode to prevent blocking stdin reads
     std::env::set_var("OWLEN_AUTO_CONSENT", "1");
 
@@ -53,6 +65,9 @@ async fn main() -> Result<()> {
     let (mut app, mut session_rx) = ChatApp::new(controller).await?;
     app.initialize_models().await?;
 
+    // Set the initial mode
+    app.set_mode(initial_mode).await;
+
     // Event infrastructure
     let cancellation_token = CancellationToken::new();
     let (event_tx, event_rx) = mpsc::unbounded_channel();