feat: merge search_by_metadata into search_notes with optional query

Make `query` optional in `search_notes` so it becomes the single search tool.
Remove `search_by_metadata` entirely — it was unreleased and redundant since
`search_notes` already supports `metadata_filters`, `tags`, and `status`.

- 🔧 `query` param is now `Optional[str] = None`
- 🛡️ Added None guards for project detection and URL resolution
- ✅ Added `no_criteria()` validation with helpful error message
- 🗑️ Deleted `search_by_metadata` tool, imports, tests, and contract entry
- 📝 Updated docs, README, and v0.19.0 release notes

Closes #605

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/cli/commands/tool.py

@@ -485,7 +485,7 @@ def search_notes(
         with force_routing(local=local, cloud=cloud):
             result = run_with_cleanup(
                 mcp_search(
-                    query=query or "",
+                    query=query or None,
                     project=project,
                     workspace=workspace,
                     search_type=search_type,