feat: add ssh_key_id to config profiles for container key mounting

- Add ssh_key_id column to ConfigProfile model and migration
- Update config profile API to accept/return ssh_key_id
- Include ssh_key_id in ResolvedProfile and resolver logic
- Mount selected SSH key into container home dir at start_instance
- Frontend config profile form with SSH key selector dropdown
- Git mount URL validation defaults to profile's SSH key

Quality gates: pytest (231 passed, 6 pre-existing), tsc --noEmit clean

apps/api/src/services/ssh_keys.py

@@ -19,17 +19,18 @@ def _get_fernet() -> Fernet:
     return Fernet(key)
 
 
-def prepare_ssh_key_files(instance_dir: str, ssh_key) -> str:
+def prepare_ssh_key_files(instance_dir: str, ssh_key, subdir: str = ".ssh") -> str:
     """Decrypt and write SSH key files to instance directory for container mounting.
 
     Args:
         instance_dir: Path to instance directory
         ssh_key: SSHKey model instance with encrypted private key
+        subdir: Subdirectory within instance_dir to write to (default: ".ssh")
 
     Returns:
         Path to the .ssh directory
     """
-    ssh_dir = Path(instance_dir) / ".ssh"
+    ssh_dir = Path(instance_dir) / subdir
     ssh_dir.mkdir(parents=True, exist_ok=True)
 
     # Decrypt private key