use native Claude Code installer (curl | bash), check prerequisites in birth

- install-cli.sh: suggest curl -sL https://claude.ai/install.sh | bash
- birth.sh: check colony, colony-agent, claude are in PATH before starting
- Fail fast with clear error messages if anything is missing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/birth.sh

@@ -20,6 +20,18 @@ case "${NAME}" in
         ;;
 esac
 
+# Check prerequisites
+for cmd in colony colony-agent; do
+    if ! command -v "${cmd}" &>/dev/null; then
+        echo "ERROR: ${cmd} not found. Run: sudo bash scripts/install-cli.sh"
+        exit 1
+    fi
+done
+if ! command -v claude &>/dev/null; then
+    echo "ERROR: claude not found. Install with: curl -sL https://claude.ai/install.sh | bash"
+    exit 1
+fi
+
 echo "=== Birthing agent: ${NAME} ==="
 echo "Instruction: ${INSTRUCTION}"
 echo "Colony: ${COLONY_URL}"