fix: run tool terminal sessions as container user instead of root

- Remove compose-level user: 0:0 override from manifest_compiler.py so the
  entrypoint can start as root, fix mount ownership, and drop privileges to
  the container user internally.
- Add get_manifest_container_user() helper to resolve the manifest-declared
  container user (with uid:gid fallback).
- Pass container user through TerminalSession, TerminalManager, and the
  terminal WebSocket handler so docker exec is invoked with --user <user>.
- Update and add unit tests for the manifest compiler and terminal session.
- Record the additional root-user fix in the fix-pi-container-mount-permissions
  OpenSpec change/tasks.

Quality gates: pytest tests/unit/ (226 passed), pytest tests/services/test_terminal_manager_multi.py (7 passed), ruff check on changed files (clean), mypy on changed files (clean)

apps/api/src/services/build/manifest_compiler.py

@@ -408,7 +408,6 @@ def compile_compose(manifest: dict, variables: dict[str, Any]) -> str:
         Docker Compose YAML content.
     """
     runtime = manifest.get("runtime", {})
-    user = manifest.get("user")
     interface_type = manifest["interface_type"]
     home_dir = get_manifest_home_dir(manifest)
 
@@ -436,12 +435,11 @@ def compile_compose(manifest: dict, variables: dict[str, Any]) -> str:
     else:
         service["working_dir"] = f"{home_dir}/{workspace_name}"
 
-    # The entrypoint starts as root (Dockerfile does not set USER) so it can
-    # fix mount ownership. It drops privileges to the container user internally
-    # before exec-ing the real command, so do not set compose-level user
-    # override here.
-    if user:
-        service["user"] = "0:0"
+    # The Dockerfile does not set USER so the entrypoint starts as root,
+    # fixes mount ownership, and drops privileges to the container user
+    # internally. Do not set a compose-level user override: that would pin
+    # the container metadata to root and make docker exec sessions run as
+    # root even after the entrypoint drops privileges.
 
     # Ports for web tools
     default_port = manifest.get("default_port")
@@ -568,6 +566,35 @@ def get_manifest_home_dir(manifest: dict) -> str:
     return "/root"
 
 
+def get_manifest_container_user(manifest: dict) -> str | None:
+    """Resolve the container user identifier from a manifest.
+
+    Returns the user name when available so that docker exec sessions can
+    attach as the container user instead of defaulting to root. Falls back
+    to ``uid:gid`` when a name is absent but numeric ids are present.
+
+    Args:
+        manifest: Fully resolved manifest JSON.
+
+    Returns:
+        User name (e.g. ``user``), ``uid:gid`` string, or None when the
+        manifest does not declare a user.
+    """
+    user = manifest.get("user")
+    if not user:
+        return None
+
+    name = user.get("name")
+    if name:
+        return name
+
+    uid = user.get("uid")
+    gid = user.get("gid")
+    if uid is not None and gid is not None:
+        return f"{uid}:{gid}"
+    return None
+
+
 def compute_image_tag(tool_name: str, manifest: dict) -> str:
     """Compute a deterministic image tag from manifest content.