Fix SSH commit signing in devcontainer when host uses 1Password (#25)

When the host configures `gpg.ssh.program` to use 1Password's
`op-ssh-sign` binary, commit signing fails inside the container
because that macOS-specific binary doesn't exist.

DevContainers automatically forward SSH_AUTH_SOCK from the host,
so the SSH agent (including 1Password's) is already available.
By overriding `gpg.ssh.program` to `/usr/bin/ssh-keygen` in the
container's local gitconfig (which is included after the host
config), signing uses the standard ssh-keygen against the
forwarded agent instead of the missing 1Password binary.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

post_install.py

@@ -199,6 +199,9 @@ node_modules/
 
 [diff]
     colorMoved = default
+
+[gpg "ssh"]
+    program = /usr/bin/ssh-keygen
 """
     local_gitconfig.write_text(local_config, encoding="utf-8")
     print(f"[post_install] Local git config created: {local_gitconfig}", file=sys.stderr)