feat(desktop): discover remote agent harnesses

[dspury]

Summary

• add read-only Tauri commands to list configured SSH hosts and probe a selected host for available agent harnesses
• reuse the existing local harness catalog to derive remote probe targets instead of maintaining a separate list
• add a conservative SSH config parser and strict host-key, batch-mode probing
• expose typed desktop bridge APIs for remote host discovery

Why

Buzz can already discover harnesses on the local machine. This adds the same discovery shape for agents that are already running on another machine, without installing software, writing remote state, or collecting credentials.

Validation

• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --check
• cargo test --manifest-path desktop/src-tauri/Cargo.toml remote_probe --lib
• cargo test --manifest-path desktop/src-tauri/Cargo.toml ssh_config --lib
• just desktop-tauri-clippy
• cd desktop && pnpm typecheck
• git diff --check upstream/main..HEAD