fix(openviking): add missing /agent/{agent}/ segment to memory URI — fixes #36969

_build_memory_uri produced URIs of the form:
  viking://user/{user}/memories/{subdir}/mem_{slug}.md

The /agent/{agent}/ segment was missing, causing every agent under
the same user to write into the same flat namespace. In multi-agent
deployments agents silently overwrite each other's memories and
vector retrieval cross-pollinates results.

self._agent was already populated correctly (from OPENVIKING_AGENT
env var, default 'hermes') and sent via X-OpenViking-Agent header —
it was simply not interpolated into the URI.

Fix: add the missing segment so URIs follow the documented shape:
  viking://user/{user}/agent/{agent}/memories/{subdir}/mem_{slug}.md

Tests: 4 new regression tests in TestOpenVikingMemoryUriBuilder,
13/13 passed (9 existing + 4 new).

plugins/memory/openviking/__init__.py

@@ -627,9 +627,9 @@ class OpenVikingMemoryProvider(MemoryProvider):
             logger.warning("OpenViking session commit failed: %s", e)
 
     def _build_memory_uri(self, subdir: str) -> str:
-        """Build a viking:// memory URI under the configured user/subdir."""
+        """Build a viking:// memory URI under the configured user/agent/subdir."""
         slug = uuid.uuid4().hex[:12]
-        return f"viking://user/{self._user}/memories/{subdir}/mem_{slug}.md"
+        return f"viking://user/{self._user}/agent/{self._agent}/memories/{subdir}/mem_{slug}.md"
 
     def on_memory_write(
         self,