`feat: add context manager support to EmbeddingStore`

[VaibhavDangaich]

Description:
Problem

EmbeddingStore in code_review_graph/embeddings.py:357-388 opens a SQLite
connection in init but does not implement enter/exit. If an
exception occurs during usage before .close() is called, the connection leaks.

store = EmbeddingStore(db_path)
store.embed_nodes(nodes) # if this raises, connection is never closed
store.close()

Expected Behavior

Add context manager support, consistent with GraphStore which already
implements this pattern:

class EmbeddingStore:
...
def enter(self):
return self

  def __exit__(self, *exc):                                                 
      self.close()

This enables safe usage:
with EmbeddingStore(db_path) as store:
store.embed_nodes(nodes)

Impact

Connection leaks under error conditions, especially relevant for long-running
processes like watch mode or MCP server.