Integrate Codex patterns: task scheduler, TUI, sessions, compaction

Add Codex-inspired patterns for improved agent architecture:

**Task Scheduler (Phase 1 & 4)**
- New Task enum with Job, ToolExec, Background variants
- TaskHandler trait for custom background tasks
- Scheduler.spawn_subtask() and spawn_batch() for parallel execution
- Worker executes multiple tools in parallel via futures::join_all

**Tool Approval System (Phase 6)**
- Tool.requires_approval() method (default false for sandboxed tools)
- HttpTool marked as requiring approval (external network)
- MCP protocol annotations: destructive_hint, side_effects_hint

**Ratatui TUI CLI (Phase 2)**
- Replace blocking stdin with event-driven TUI
- ChatComposer with history navigation and tab completion
- ApprovalOverlay modal with y/n/a keyboard shortcuts
- Raw mode with proper terminal cleanup

**Session/Turn Model (Phase 3)**
- Session, Thread, Turn structs for conversation tracking
- Submission enum for user input, approvals, undo, interrupt
- UndoManager with checkpoint-based undo/redo

**Context Compaction (Phase 5)**
- ContextMonitor with token estimation and threshold checks
- CompactionStrategy: Summarize, Truncate, MoveToWorkspace
- ContextCompactor writes summaries to workspace daily logs

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/channels/mod.rs

@@ -4,14 +4,14 @@
 //! and convert them to a unified message format for the agent to process.
 
 mod channel;
-mod cli;
+pub mod cli;
 mod http;
 mod manager;
 mod slack;
 mod telegram;
 
 pub use channel::{Channel, IncomingMessage, MessageStream, OutgoingResponse};
-pub use cli::CliChannel;
+pub use cli::{SimpleCliChannel as CliChannel, TuiChannel};
 pub use http::HttpChannel;
 pub use manager::ChannelManager;
 pub use slack::SlackChannel;