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/agent/mod.rs

@@ -6,17 +6,31 @@
 //! - Tool invocation with safety
 //! - Self-repair for stuck jobs
 //! - Proactive heartbeat execution
+//! - Turn-based session management with undo
+//! - Context compaction for long conversations
 
 mod agent_loop;
+pub mod compaction;
+pub mod context_monitor;
 mod heartbeat;
 mod router;
 mod scheduler;
 mod self_repair;
+pub mod session;
+pub mod submission;
+pub mod task;
+pub mod undo;
 mod worker;
 
 pub use agent_loop::Agent;
+pub use compaction::{CompactionResult, ContextCompactor};
+pub use context_monitor::{CompactionStrategy, ContextBreakdown, ContextMonitor};
 pub use heartbeat::{HeartbeatConfig, HeartbeatResult, HeartbeatRunner, spawn_heartbeat};
 pub use router::{MessageIntent, Router};
 pub use scheduler::Scheduler;
 pub use self_repair::{RepairResult, RepairTask, SelfRepair, StuckJob};
+pub use session::{Session, Thread, ThreadState, Turn, TurnState};
+pub use submission::{Submission, SubmissionResult};
+pub use task::{Task, TaskContext, TaskHandler, TaskOutput, TaskStatus};
+pub use undo::{Checkpoint, UndoManager};
 pub use worker::Worker;