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/config_profile_resolver.py

@@ -51,6 +51,7 @@ class ResolvedProfile:
     mounts: dict[str, ResolvedMount] = field(default_factory=dict)
     git_mounts: list[dict[str, Any]] = field(default_factory=list)
     files: dict[str, str] = field(default_factory=dict)
+    ssh_key_id: uuid.UUID | None = None
     env_overrides: dict[str, str] = field(default_factory=dict)
     hint_overrides: dict[str, str] = field(default_factory=dict)
     file_overrides: dict[str, str] = field(default_factory=dict)
@@ -318,6 +319,9 @@ async def _resolve_profile_recursive(
         result.git_mounts = _merge_git_mounts(
             result.git_mounts, included.git_mounts, included.profile_name
         )
+        # Later included profile's SSH key wins
+        if included.ssh_key_id is not None:
+            result.ssh_key_id = included.ssh_key_id
 
     # Apply the profile's own settings (selected profile overrides includes)
     result.env_vars = _merge_env_vars(
@@ -349,6 +353,9 @@ async def _resolve_profile_recursive(
         profile.git_mounts or [],
         profile.name,
     )
+    # Own SSH key overrides any inherited one
+    if profile.ssh_key_id is not None:
+        result.ssh_key_id = profile.ssh_key_id
     return result
 
 
@@ -564,4 +571,5 @@ def resolved_profile_to_dict(resolved: ResolvedProfile) -> dict[str, Any]:
         },
         "git_mounts": resolved.git_mounts,
         "included_profiles": resolved.included_profiles,
+        "ssh_key_id": str(resolved.ssh_key_id) if resolved.ssh_key_id else None,
     }

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