docs: surface auth vault commands in README

[hiSandog]

Summary

• add an Authentication Vault command section to the README command reference
• include the core agent-browser auth ... subcommands in one place
• clarify that these commands are easy to miss when scanning the command list first

Why

The README already describes the auth vault in the security/authentication sections, but the command reference did not list the auth subcommands. That makes the feature harder to discover for users who start from the commands section.

Validation

• confirmed the CLI source defines the agent-browser auth subcommands in cli/src/output.rs and cli/src/commands.rs
• reviewed the README diff to ensure the new section is narrow and consistent with existing command documentation

[vercel]

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