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-core/src/config.rs

@@ -1,3 +1,4 @@
+use crate::mode::ModeConfig;
 use crate::provider::ProviderConfig;
 use crate::Result;
 use serde::{Deserialize, Serialize};
@@ -38,6 +39,9 @@ pub struct Config {
     /// Per-tool configuration toggles
     #[serde(default)]
     pub tools: ToolSettings,
+    /// Mode-specific tool availability configuration
+    #[serde(default)]
+    pub modes: ModeConfig,
 }
 
 impl Default for Config {
@@ -59,6 +63,7 @@ impl Default for Config {
             privacy: PrivacySettings::default(),
             security: SecuritySettings::default(),
             tools: ToolSettings::default(),
+            modes: ModeConfig::default(),
         }
     }
 }