docs: rewrite for clarity and supply chain focus

[Pierozi]

Summary

• Emphasize supply chain attack prevention as the primary threat throughout
• Add bun profile to all documentation, completions, and aliases (sxb)
• Remove unsupported allow_domains/deny_domains from config examples
• Consolidate redundant sections and apply humanizer rules for direct, human voice

Test plan

• Verify all profiles (base, online, localhost, rust, bun, claude, gpg) are documented consistently
• Confirm sxb alias works in all 3 shells (zsh, bash, fish)
• Check that deny_read list in README matches base.toml
• Ensure no references to removed features (allow_domains, non-existent profiles)