Update lethal trifecta landing page copy

[lpcox]

Update the lethal trifecta copy in docs/src/components/landing/SafeByDefaultTrifecta.astro with the following replacements:

• Replace “gh-aw puts six layers between the agent and your repository, and three of them each block one leg of it.” with “gh-aw defends in depth through six security layers that prevent agents from leaking private data.”
• Replace “Secrets, tokens, private repos” with “Secrets and credentials”.
• Replace “Credential isolation” with “Isolated sandbox”.
• Replace “Sandboxed agent” with “Agent firewall”.
• Replace “Issues, PR comments, web pages” with “Issues, PR comments”.
• Replace “Network, APIs, writes” with “Web pages, APIs, writes”.