feat(hooks): add cross-platform completion notification hook

Alerts user when Claude stops (task complete, needs input, etc.)
with context about what completed. Works on Mac (osascript alert),
Linux (notify-send/zenity), and Windows (PowerShell MessageBox).

Follows same optional install pattern as statusline:
- Prompts in interactive mode if existing hook detected
- --force-notify to replace existing
- --no-notify to skip installation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hooks/gsd-notify.sh

@@ -0,0 +1,69 @@
+#!/bin/bash
+# GSD Completion Notification Hook
+# Cross-platform alert when Claude stops (task complete, needs input, etc.)
+
+input=$(cat)
+session_id=$(echo "$input" | jq -r '.session_id // empty')
+cwd=$(echo "$input" | jq -r '.cwd // empty')
+transcript_path=$(echo "$input" | jq -r '.transcript_path // empty')
+
+# Extract project name
+project="Claude Code"
+if [[ -n "$cwd" ]]; then
+  project=$(basename "$cwd")
+fi
+
+# Try to get context from GSD state file
+message=""
+state_file="$cwd/.planning/STATE.md"
+if [[ -f "$state_file" ]]; then
+  phase=$(grep -m1 "^current_phase:" "$state_file" 2>/dev/null | sed 's/^current_phase: *//')
+  status=$(grep -m1 "^status:" "$state_file" 2>/dev/null | sed 's/^status: *//')
+  if [[ -n "$phase" && -n "$status" ]]; then
+    message="Phase $phase: $status"
+  fi
+fi
+
+# Fallback: check todo list for current/completed task
+if [[ -z "$message" && -n "$session_id" ]]; then
+  todo_file=$(ls -t "$HOME/.claude/todos/${session_id}"*.json 2>/dev/null | head -1)
+  if [[ -f "$todo_file" ]]; then
+    # Get most recently completed task, or in-progress task
+    completed=$(jq -r '[.[] | select(.status=="completed")] | last | .content // empty' "$todo_file" 2>/dev/null)
+    if [[ -n "$completed" ]]; then
+      message="Completed: $completed"
+    else
+      in_progress=$(jq -r '.[] | select(.status=="in_progress") | .content' "$todo_file" 2>/dev/null | head -1)
+      if [[ -n "$in_progress" ]]; then
+        message="Paused: $in_progress"
+      fi
+    fi
+  fi
+fi
+
+# Fallback: generic message
+if [[ -z "$message" ]]; then
+  message="Ready for input"
+fi
+
+# Send notification based on OS
+case "$(uname -s)" in
+  Darwin)
+    osascript -e "display alert \"GSD: $project\" message \"$message\" as informational" &>/dev/null &
+    ;;
+  Linux)
+    if command -v notify-send &>/dev/null; then
+      notify-send "GSD: $project" "$message" --urgency=normal
+    elif command -v zenity &>/dev/null; then
+      zenity --info --title="GSD: $project" --text="$message" &
+    fi
+    ;;
+  MINGW*|CYGWIN*|MSYS*)
+    # Windows via PowerShell
+    if command -v powershell.exe &>/dev/null; then
+      powershell.exe -Command "Add-Type -AssemblyName System.Windows.Forms; [System.Windows.Forms.MessageBox]::Show('$message', 'GSD: $project', 'OK', 'Information')" &>/dev/null &
+    fi
+    ;;
+esac
+
+exit 0