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/tools/tool.rs

@@ -148,6 +148,18 @@ pub trait Tool: Send + Sync {
         true
     }
 
+    /// Whether this tool requires explicit user approval before execution.
+    ///
+    /// Returns false by default since most tools run in a sandboxed/virtualized
+    /// environment. Only tools that make external network calls or perform
+    /// destructive operations should return true.
+    ///
+    /// When true, the agent will prompt the user for confirmation before
+    /// executing this tool.
+    fn requires_approval(&self) -> bool {
+        false
+    }
+
     /// Get the tool schema for LLM function calling.
     fn schema(&self) -> ToolSchema {
         ToolSchema {