feat: add text clipboard copy/read with mouse selection support

Add cross-platform text clipboard operations (macOS, Linux, Windows)
with OSC 52 support for SSH/tmux environments. Wire up onMouseUp and
Ctrl+Y in the TUI to copy selected text to the system clipboard via
OpenTUI's renderer selection API.

src/constants/plan-approval.ts

@@ -0,0 +1,58 @@
+/**
+ * Plan Approval Constants
+ *
+ * Options and configuration for the plan approval prompt,
+ * modeled after Claude Code's plan approval flow.
+ */
+
+/** How edits should be handled after plan approval */
+export type PlanEditMode =
+  | "auto_accept_clear"   // Clear context and auto-accept edits
+  | "auto_accept"         // Auto-accept edits
+  | "manual_approve"      // Manually approve each edit
+  | "feedback";           // User provides feedback/changes
+
+/** A selectable option in the plan approval modal */
+export interface PlanApprovalOption {
+  key: string;
+  label: string;
+  description: string;
+  editMode: PlanEditMode;
+  shortcut?: string;
+}
+
+/** Available plan approval options, matching Claude Code's pattern */
+export const PLAN_APPROVAL_OPTIONS: PlanApprovalOption[] = [
+  {
+    key: "1",
+    label: "Yes, clear context and auto-accept edits",
+    description: "Approve plan, clear conversation context, and auto-accept all file edits",
+    editMode: "auto_accept_clear",
+    shortcut: "shift+tab",
+  },
+  {
+    key: "2",
+    label: "Yes, auto-accept edits",
+    description: "Approve plan and auto-accept all file edits without prompting",
+    editMode: "auto_accept",
+  },
+  {
+    key: "3",
+    label: "Yes, manually approve edits",
+    description: "Approve plan but require manual approval for each file edit",
+    editMode: "manual_approve",
+  },
+  {
+    key: "4",
+    label: "Type here to tell CodeTyper what to change",
+    description: "Provide feedback or modifications to the plan",
+    editMode: "feedback",
+  },
+];
+
+/** Maximum visible options before scrolling */
+export const PLAN_APPROVAL_MAX_VISIBLE_OPTIONS = 4;
+
+/** Footer help text for plan approval modal */
+export const PLAN_APPROVAL_FOOTER_TEXT =
+  "ctrl-g to edit in editor";