feat(knowledge): auto-maintain index.md, improve import UX, fix embedding provider

- Auto-rebuild knowledge/index.md from the real directory tree on
  create/import so it never drifts or loses documents (no longer relies
  on the agent hand-writing it).
- Auto-open the created/imported document in the tree after success.
- Add create_document status message, shorten EN action buttons, and
  localize the "insert template" content.
- Show filename for protected system files (index.md/log.md) in the tree
  instead of their H1 heading.
- Reuse a shared embedding-provider factory so knowledge index sync also
  gets vectors instead of degrading to keyword-only search.

agent/memory/embedding/__init__.py

@@ -16,6 +16,7 @@ from agent.memory.embedding.provider import (
     OpenAIEmbeddingProvider,
     create_embedding_provider,
 )
+from agent.memory.embedding.factory import create_default_embedding_provider
 from agent.memory.embedding.rebuild import (
     RebuildResult,
     clear_index,
@@ -33,6 +34,7 @@ __all__ = [
     "EmbeddingProvider",
     "OpenAIEmbeddingProvider",
     "create_embedding_provider",
+    "create_default_embedding_provider",
     "RebuildResult",
     "clear_index",
     "rebuild_in_process",