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/lib.rs

@@ -12,6 +12,7 @@ pub mod encryption;
 pub mod formatting;
 pub mod input;
 pub mod mcp;
+pub mod mode;
 pub mod model;
 pub mod provider;
 pub mod router;
@@ -34,6 +35,7 @@ pub use encryption::*;
 pub use formatting::*;
 pub use input::*;
 pub use mcp::*;
+pub use mode::*;
 pub use model::*;
 pub use provider::*;
 pub use router::*;