fix: mount config profile files individually instead of replacing directories

The previous sorting fix exposed a deeper bug: ResolvedMount always
mounted its staging directory as a single bind mount. When a config
profile mount targeted /workspace/x/y and contained a single file
z.json, the staging directory (containing only z.json) replaced the
ENTIRE /workspace/x/y directory, hiding all sibling files from git
repo mounts.

- Change apply_resolved_profile to mount each file individually:
  - source: staging_dir/relative_path
  - target: expanded_target/relative_path
  - Sibling files from other mounts are preserved.
  - Empty mounts produce no volume entries.

- Keep volume sorting (parent paths before child paths) which is
  still necessary for directory mounts and ensures parent dirs exist
  before file mounts inside them.

- Add 4 unit tests for file-level mount behavior.

Quality gates: pytest (218 passed, 6 pre-existing), tsc --noEmit (clean)

apps/api/tests/unit/test_config_profile_resolver.py

@@ -6,6 +6,9 @@ from src.models.config_profile import ConfigProfile, ConfigProfileInclude
 from src.services.config_profile_resolver import (
     ConfigProfileCycleError,
     ConfigProfileNotFoundError,
+    ResolvedMount,
+    ResolvedProfile,
+    apply_resolved_profile,
     check_include_cycle,
     resolve_profile,
     _merge_env_vars,
@@ -479,6 +482,81 @@ class TestResolveProfile:
             await resolve_profile(db_session, uuid.uuid4())
 
 
+class TestApplyResolvedProfile:
+    """Unit tests for apply_resolved_profile file-level mount behavior."""
+
+    def test_mounts_individual_files_not_directory(self, tmp_path) -> None:
+        """Each file in a ResolvedMount should be mounted individually, not the staging dir."""
+        resolved = ResolvedProfile(
+            profile_id=uuid.uuid4(),
+            profile_name="test",
+            mounts={
+                "/app": ResolvedMount(
+                    target="/app",
+                    mode="rw",
+                    files={"config.json": '{"key": "value"}', "nested/file.txt": "hello"},
+                )
+            },
+        )
+        env, files, volumes, hints = apply_resolved_profile(str(tmp_path), resolved)
+
+        assert len(volumes) == 2
+        targets = {v["target"] for v in volumes}
+        assert "/app/config.json" in targets
+        assert "/app/nested/file.txt" in targets
+        # No directory-level mount
+        assert "/app" not in targets
+
+    def test_file_mount_preserves_sibling_files(self, tmp_path) -> None:
+        """File-level mounts should not hide sibling files from other mounts."""
+        resolved = ResolvedProfile(
+            profile_id=uuid.uuid4(),
+            profile_name="test",
+            mounts={
+                "/workspace/x/y": ResolvedMount(
+                    target="/workspace/x/y",
+                    mode="rw",
+                    files={"z.json": "override"},
+                )
+            },
+        )
+        env, files, volumes, hints = apply_resolved_profile(str(tmp_path), resolved)
+
+        assert len(volumes) == 1
+        assert volumes[0]["target"] == "/workspace/x/y/z.json"
+        assert volumes[0]["source"].endswith("z.json")
+
+    def test_empty_mount_produces_no_volumes(self, tmp_path) -> None:
+        """A mount with no files should not produce any volume entries."""
+        resolved = ResolvedProfile(
+            profile_id=uuid.uuid4(),
+            profile_name="test",
+            mounts={
+                "/app": ResolvedMount(target="/app", mode="rw", files={})
+            },
+        )
+        env, files, volumes, hints = apply_resolved_profile(str(tmp_path), resolved)
+        assert volumes == []
+
+    def test_home_expansion_in_file_mount_target(self, tmp_path) -> None:
+        """~ in mount target should be expanded to home_dir for file mounts."""
+        resolved = ResolvedProfile(
+            profile_id=uuid.uuid4(),
+            profile_name="test",
+            mounts={
+                "~/.config": ResolvedMount(
+                    target="~/.config",
+                    mode="rw",
+                    files={"app.toml": "setting = 1"},
+                )
+            },
+        )
+        env, files, volumes, hints = apply_resolved_profile(
+            str(tmp_path), resolved, home_dir="/home/user"
+        )
+        assert volumes[0]["target"] == "/home/user/.config/app.toml"
+
+
 class TestCheckIncludeCycle:
     """Unit tests for include cycle checking."""