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-tui/src/ui.rs

@@ -1298,6 +1298,20 @@ fn render_status(frame: &mut Frame<'_>, area: Rect, app: &ChatApp) {
         ));
     }
 
+    // Add operating mode indicator
+    let operating_mode = app.get_mode();
+    let (op_mode_text, op_mode_color) = match operating_mode {
+        owlen_core::mode::Mode::Chat => (" 💬 CHAT", Color::Blue),
+        owlen_core::mode::Mode::Code => (" 💻 CODE", Color::Magenta),
+    };
+    spans.push(Span::styled(
+        op_mode_text,
+        Style::default()
+            .fg(Color::Black)
+            .bg(op_mode_color)
+            .add_modifier(Modifier::BOLD),
+    ));
+
     spans.push(Span::styled("  ", Style::default().fg(theme.text)));
     spans.push(Span::styled(help_text, Style::default().fg(theme.info)));
 
@@ -1861,6 +1875,12 @@ fn render_help(frame: &mut Frame<'_>, app: &ChatApp) {
             Line::from("  :w [name]          → alias for :save"),
             Line::from("  :load, :o, :open   → browse and load saved sessions"),
             Line::from("  :sessions, :ls     → browse saved sessions"),
+            // New mode and tool commands added in phases 0‑5
+            Line::from("  :code               → switch to code mode (CLI: owlen --code)"),
+            Line::from("  :mode <chat|code>   → change current mode explicitly"),
+            Line::from("  :tools              → list tools available in the current mode"),
+            Line::from("  :agent status       → show agent configuration and iteration info"),
+            Line::from("  :stop-agent         → abort a running ReAct agent loop"),
         ],
         4 => vec![
             // Sessions