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/__init__.py

@@ -7,7 +7,7 @@ conversation history persistence (SQLite).
 
 from agent.memory.manager import MemoryManager
 from agent.memory.config import MemoryConfig, get_default_memory_config, set_global_memory_config
-from agent.memory.embedding import create_embedding_provider
+from agent.memory.embedding import create_embedding_provider, create_default_embedding_provider
 from agent.memory.conversation_store import ConversationStore, get_conversation_store
 from agent.memory.summarizer import ensure_daily_memory_file
 
@@ -17,6 +17,7 @@ __all__ = [
     'get_default_memory_config',
     'set_global_memory_config',
     'create_embedding_provider',
+    'create_default_embedding_provider',
     'ConversationStore',
     'get_conversation_store',
     'ensure_daily_memory_file',