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/safety/leak_detector.rs

@@ -299,12 +299,11 @@ impl LeakDetector {
 
         // Scan each header value
         for (name, value) in headers {
-            self.scan_and_clean(value).map_err(|e| {
-                LeakDetectionError::SecretLeakBlocked {
+            self.scan_and_clean(value)
+                .map_err(|e| LeakDetectionError::SecretLeakBlocked {
                     pattern: format!("header:{}", name),
                     preview: e.to_string(),
-                }
-            })?;
+                })?;
         }
 
         // Scan body if present and valid UTF-8
@@ -688,7 +687,10 @@ mod tests {
         // Attempt to exfiltrate in custom header
         let result = detector.scan_http_request(
             "https://api.example.com/data",
-            &[("X-Custom".to_string(), "ghp_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx".to_string())],
+            &[(
+                "X-Custom".to_string(),
+                "ghp_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx".to_string(),
+            )],
             None,
         );
         assert!(result.is_err());
@@ -700,11 +702,7 @@ mod tests {
 
         // Attempt to exfiltrate in request body
         let body = b"{\"stolen\": \"sk-proj-test1234567890abcdefghij\"}";
-        let result = detector.scan_http_request(
-            "https://api.example.com/webhook",
-            &[],
-            Some(body),
-        );
+        let result = detector.scan_http_request("https://api.example.com/webhook", &[], Some(body));
         assert!(result.is_err());
     }
 }