refactor: remove ChromaDB dependency gating from status and preflight

Embed status no longer checks for chromadb import — vec0 runs in
paperforge.db and doesn't need the ChromaDB library. The preflight
check also no longer requires chromadb since vec0 backends do not
depend on it. The chroma_backend module remains for legacy test
compatibility but no longer gates any production build/status path.

paperforge/commands/embed.py

@@ -122,10 +122,6 @@ def run(args: argparse.Namespace) -> int:
             import openai  # noqa: F401
         except ImportError:
             _dep_missing.append("openai")
-        try:
-            import chromadb  # noqa: F401
-        except ImportError:
-            _dep_missing.append("chromadb")
         write_vector_runtime(
             vault,
             enabled=bool(status.get("mode", "")),

paperforge/embedding/preflight.py

@@ -17,16 +17,6 @@ def _preflight_check(vault: Path, settings: dict | None = None) -> dict:
             "fix": 'Run: pip install "paperforge[vector]"',
         }
 
-    # 2. chromadb package
-    try:
-        import chromadb  # noqa: F401
-    except ImportError:
-        return {
-            "ok": False,
-            "error": "chromadb is not installed",
-            "fix": 'Run: pip install "paperforge[vector]"',
-        }
-
     # 3. API key
     api_key = None
     if settings: