Add automated update script for claude-cli package

Create update.sh script that fetches the latest claude-code version
and SHA256 hashes from Homebrew cask and automatically updates the
Nix package definition.

Features:
- Fetches version and all 4 platform SHA256 hashes from Homebrew
- Updates default.nix in-place using awk for reliable parsing
- Validates extracted data before updating
- Supports --dry-run mode to preview changes
- Supports --help flag for usage information
- Idempotent (safe to run multiple times)

Update README to document the automated update method as the
recommended approach, with manual update as a fallback option.

packages/claude-cli/README.md

@@ -8,9 +8,33 @@ The official `claude-code` package in nixpkgs tries to fetch from npm registry,
 
 ## Updating to a New Version
 
-When a new version of Claude Code is released, follow these steps:
+### Automated Update (Recommended)
 
-### 1. Find the Latest Version and Hashes
+Run the update script to automatically fetch and update to the latest version:
+
+```bash
+cd packages/claude-cli
+./update.sh
+```
+
+The script will:
+- Fetch the latest version from Homebrew cask
+- Update version and all SHA256 hashes in default.nix
+- Show you what changed
+
+For a dry-run to see what would change:
+
+```bash
+./update.sh --dry-run
+```
+
+After the script completes, follow the "Test the Build" steps below.
+
+### Manual Update
+
+If you prefer to update manually, or if the automated script fails:
+
+#### 1. Find the Latest Version and Hashes
 
 Check the Homebrew cask formula for the latest version info:
 
@@ -22,7 +46,7 @@ This will show:
 - The latest `version` number
 - SHA256 hashes for all platforms (`arm64`, `x86_64`, `x86_64_linux`, `arm64_linux`)
 
-### 2. Update default.nix
+#### 2. Update default.nix
 
 Edit `default.nix` and update:
 
@@ -47,7 +71,7 @@ Edit `default.nix` and update:
    };
    ```
 
-### 3. Test the Build
+#### 3. Test the Build
 
 Before committing, test that the package builds successfully:
 
@@ -67,7 +91,7 @@ Clean up the test build:
 rm result
 ```
 
-### 4. Deploy
+#### 4. Deploy
 
 Commit your changes and rebuild: