feat(cli): add self-update command for automatic CLI updates

[vasanth53]

Problem

Currently, NemoClaw users have no built-in way to check for or apply CLI updates. They must manually run npm install -g nemoclaw or clone the repository and pull changes.

Proposed Solution

Add a nemoclaw update command that:

1. Checks for updates - Compares current version against the latest release from npm and GitHub
2. Shows version info - Displays current vs latest version
3. Applies updates - Downloads and runs the install script non-interactively
4. Handles edge cases - Detects running from source and suggests git pull instead

Commands

# Check for updates (default behavior)
nemoclaw update

# Update without prompting
nemoclaw update --yes

# Force check even if already current
nemoclaw update --force

Output Example

$ nemoclaw update

  Checking for updates...

  Current version: 0.1.0
  Latest version:  0.2.0

  A new version is available!
  Run 'nemoclaw update --yes' to update without prompting.

Implementation Notes

• Uses built-in Node.js http/https modules (no external dependencies)
• Fetches version from both npm registry and GitHub releases (uses higher version)
• Detects running from source and suggests git pull instead of install script
• Downloads install script with integrity verification before execution

Status

Implemented in #644