feat: implement execution control (pause/resume/abort) for agent mode

Adds execution control system per GitHub issue #113:
- Ctrl+P: Toggle pause/resume during agent execution
- Ctrl+Z: Abort with rollback (undo file changes)
- Ctrl+Shift+S: Toggle step-by-step mode
- Enter: Advance one step when in step mode

New files:
- src/types/execution-control.ts: Type definitions
- src/services/execution-control.ts: Control implementation with rollback
- src/constants/execution-control.ts: Keyboard shortcuts and messages

Modified:
- agent-stream.ts: Integrated execution control into agent loop
- message-handler.ts: Added control functions and callbacks
- app.tsx: Added keyboard shortcut handlers
- help-content.ts: Added help topics for new shortcuts

Closes #113

src/constants/execution-control.ts

@@ -0,0 +1,53 @@
+/**
+ * Execution Control Constants
+ *
+ * Constants for agent execution control (pause/resume/abort).
+ */
+
+/**
+ * Keyboard shortcuts for execution control
+ */
+export const EXECUTION_CONTROL_KEYS = {
+  /** Toggle pause/resume (Ctrl+P) */
+  TOGGLE_PAUSE: "ctrl+p",
+  /** Abort execution (Ctrl+C) */
+  ABORT: "ctrl+c",
+  /** Abort with rollback (Ctrl+Z) */
+  ABORT_WITH_ROLLBACK: "ctrl+z",
+  /** Toggle step mode (Ctrl+Shift+S) */
+  TOGGLE_STEP_MODE: "ctrl+shift+s",
+  /** Advance one step in step mode (Enter when waiting) */
+  STEP: "return",
+} as const;
+
+/**
+ * Execution control status messages
+ */
+export const EXECUTION_CONTROL_MESSAGES = {
+  PAUSED: "⏸ Execution paused. Press Ctrl+P to resume.",
+  RESUMED: "▶ Execution resumed.",
+  STEP_MODE_ENABLED: "🚶 Step mode enabled. Press Enter to advance each tool call.",
+  STEP_MODE_DISABLED: "🏃 Step mode disabled. Execution will continue automatically.",
+  WAITING_FOR_STEP: (toolName: string): string =>
+    `⏳ Step mode: Ready to execute ${toolName}. Press Enter to continue.`,
+  ROLLBACK_ACTION: (description: string): string =>
+    `↩ Rolling back: ${description}`,
+  ROLLBACK_COMPLETE: (count: number): string =>
+    `✓ Rollback complete. ${count} action(s) undone.`,
+  ABORT_INITIATED: (rollbackCount: number): string =>
+    rollbackCount > 0
+      ? `Aborting with rollback of ${rollbackCount} action(s)...`
+      : "Aborting execution...",
+} as const;
+
+/**
+ * Help text for execution control
+ */
+export const EXECUTION_CONTROL_HELP = `
+Execution Control:
+  Ctrl+P         - Toggle pause/resume
+  Ctrl+C         - Abort execution
+  Ctrl+Z         - Abort with rollback (undo file changes)
+  Ctrl+Shift+S   - Toggle step-by-step mode
+  Enter          - Advance one step (when in step mode)
+` as const;

src/constants/help-content.ts

@@ -173,6 +173,33 @@ export const HELP_TOPICS: HelpTopic[] = [
     shortcuts: ["Ctrl+M"],
     category: "shortcuts",
   },
+  {
+    id: "shortcut-ctrlp",
+    name: "Ctrl+P",
+    shortDescription: "Pause/Resume",
+    fullDescription:
+      "Toggle pause/resume during agent execution. When paused, tool calls are suspended until resumed.",
+    shortcuts: ["Ctrl+P"],
+    category: "shortcuts",
+  },
+  {
+    id: "shortcut-ctrlz",
+    name: "Ctrl+Z",
+    shortDescription: "Abort with rollback",
+    fullDescription:
+      "Abort current operation and rollback file changes made during this execution. Use to undo unwanted modifications.",
+    shortcuts: ["Ctrl+Z"],
+    category: "shortcuts",
+  },
+  {
+    id: "shortcut-ctrlshifts",
+    name: "Ctrl+Shift+S",
+    shortDescription: "Toggle step mode",
+    fullDescription:
+      "Enable step-by-step mode where you can advance one tool call at a time. Press Enter to execute each tool.",
+    shortcuts: ["Ctrl+Shift+S"],
+    category: "shortcuts",
+  },
 ];
 
 export const getTopicsByCategory = (category: HelpCategory): HelpTopic[] =>