SSH config: add IgnoreUnknown UseKeychain during onboarding

[bordumb]

Problem

auths init writes UseKeychain yes to ~/.ssh/config under a Host * block. This is a macOS-specific OpenSSH option. If the user's SSH version doesn't recognize it, all git+SSH operations fail:

/Users/.../.ssh/config: line 7: Bad configuration option: usekeychain
/Users/.../.ssh/config: terminating, 1 bad configuration options
fatal: Could not read from remote repository.

Fix

1. auths init (onboarding)

When writing the SSH config, prepend IgnoreUnknown UseKeychain on the same Host * block:

Host *
  IgnoreUnknown UseKeychain
  AddKeysToAgent yes
  UseKeychain yes
  IdentityFile ~/.ssh/id_ed25519_...

This tells SSH to silently skip UseKeychain if unsupported, rather than failing.

2. auths doctor (diagnostics)

auths doctor should check for this condition:

• If ~/.ssh/config contains UseKeychain without a preceding IgnoreUnknown UseKeychain, flag it as a warning
• Print the location of the SSH config and suggest adding the directive
• Users who break their auths setup will likely reach for auths doctor first, so this is an important diagnostic to surface

Context

Discovered while dogfooding the @auths-dev/verify widget. After wiping and re-creating an identity, git push failed due to this SSH config issue.