Add Antigravity (agy) CLI as a clink client

Problem

The gemini clink client is effectively dead for individual accounts — Google migrated individuals off Gemini Code Assist to the Antigravity suite (the CLI now prints: "This client is no longer supported for Gemini Code Assist for individuals... migrate to the Antigravity suite of products"). That leaves clink without a first-class client for Google's current individual-tier CLI.

Proposal

Add Google's Antigravity CLI (agy) as a first-class clink client.

agy has two quirks that don't fit the current client abstraction:

1. Its -p/--print flag needs the prompt as the flag's own argument value (agy --print "<prompt>"), not via stdin — passing on stdin yields flag needs an argument: -p.
2. No structured/JSON output mode (as of v1.0.16) — --print writes plain text to stdout.

To support (1) cleanly without special-casing agy, I'd propose a small general prompt_to_arg client-config primitive (symmetric with the existing file-based hooks): render the prompt into a flag template for execution, while keeping a redacted copy for the returned sanitized_command/logs.

Would you be open to a PR for this? I have an implementation ready — verified against the real agy v1.0.16 binary — and can open it alongside this issue. Happy to adjust the approach (or narrow it to an agy-specific shim instead of a general primitive) to match your preferences.