Add script to update doomemacs to latest commit

Creates update-doomemacs.sh script that:
- Fetches latest commit SHA from doomemacs/doomemacs repo
- Automatically detects the default branch
- Updates both rev and sha256 in home/roles/emacs/default.nix
- Works from anywhere in the repo using git rev-parse

Also adds a flake app so it can be run with:
  nix run .#update-doomemacs

scripts/update-doomemacs.sh

@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Colors for output
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[1;33m'
+NC='\033[0m' # No Color
+
+# Configuration
+OWNER="doomemacs"
+REPO="doomemacs"
+FILE="home/roles/emacs/default.nix"
+# Use current working directory as repo root (allows running from anywhere in the repo)
+REPO_ROOT="${REPO_ROOT:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+TARGET_FILE="$REPO_ROOT/$FILE"
+
+echo -e "${GREEN}Updating DoomEmacs to latest commit...${NC}"
+
+# Check if file exists
+if [[ ! -f "$TARGET_FILE" ]]; then
+    echo -e "${RED}Error: $TARGET_FILE not found${NC}"
+    exit 1
+fi
+
+# Get the default branch first
+echo "Fetching repository information..."
+DEFAULT_BRANCH=$(curl -s "https://api.github.com/repos/$OWNER/$REPO" | jq -r '.default_branch')
+
+if [[ -z "$DEFAULT_BRANCH" ]] || [[ "$DEFAULT_BRANCH" == "null" ]]; then
+    echo -e "${RED}Error: Failed to fetch default branch${NC}"
+    exit 1
+fi
+
+# Get the latest commit SHA from GitHub
+echo "Fetching latest commit SHA from $DEFAULT_BRANCH branch..."
+LATEST_SHA=$(curl -s "https://api.github.com/repos/$OWNER/$REPO/commits/$DEFAULT_BRANCH" | jq -r '.sha')
+
+if [[ -z "$LATEST_SHA" ]] || [[ "$LATEST_SHA" == "null" ]]; then
+    echo -e "${RED}Error: Failed to fetch latest commit SHA${NC}"
+    exit 1
+fi
+
+echo -e "Latest commit: ${YELLOW}$LATEST_SHA${NC}"
+
+# Get current SHA from file
+CURRENT_SHA=$(grep -oP 'rev = "\K[^"]+' "$TARGET_FILE")
+echo -e "Current commit: ${YELLOW}$CURRENT_SHA${NC}"
+
+if [[ "$CURRENT_SHA" == "$LATEST_SHA" ]]; then
+    echo -e "${GREEN}Already up to date!${NC}"
+    exit 0
+fi
+
+# Update the rev field
+echo "Updating rev in $FILE..."
+sed -i "s/rev = \".*\"/rev = \"$LATEST_SHA\"/" "$TARGET_FILE"
+
+# Fetch the new sha256 hash using nix-prefetch
+echo "Fetching new sha256 hash..."
+NEW_SHA256=$(nix-prefetch-url --unpack "https://github.com/$OWNER/$REPO/archive/$LATEST_SHA.tar.gz" 2>/dev/null)
+
+if [[ -z "$NEW_SHA256" ]]; then
+    echo -e "${RED}Error: Failed to fetch sha256 hash${NC}"
+    # Revert the rev change
+    sed -i "s/rev = \".*\"/rev = \"$CURRENT_SHA\"/" "$TARGET_FILE"
+    exit 1
+fi
+
+# Convert to SRI hash format
+SRI_HASH=$(nix hash to-sri --type sha256 "$NEW_SHA256")
+echo -e "New sha256: ${YELLOW}$SRI_HASH${NC}"
+
+# Update the sha256 field
+sed -i "s|sha256 = \".*\"|sha256 = \"$SRI_HASH\"|" "$TARGET_FILE"
+
+echo -e "${GREEN}Successfully updated DoomEmacs!${NC}"
+echo -e "  Old commit: ${YELLOW}$CURRENT_SHA${NC}"
+echo -e "  New commit: ${YELLOW}$LATEST_SHA${NC}"
+echo -e "  New sha256: ${YELLOW}$SRI_HASH${NC}"
+echo ""
+echo "You can now rebuild your system with the updated DoomEmacs."