fix: stale agent timeout, uv venv detection, empty response after tools (#9051, #8620, #9400)

Three independent fixes:

1. Reset activity timestamp on cached agent reuse (#9051)
   When the gateway reuses a cached AIAgent for a new turn, the
   _last_activity_ts from the previous turn (possibly hours ago)
   carried over. The inactivity timeout handler immediately saw
   the agent as idle for hours and killed it.

   Fix: reset _last_activity_ts, _last_activity_desc, and
   _api_call_count when retrieving an agent from the cache.

2. Detect uv-managed virtual environments (#8620 sub-issue 1)
   The systemd unit generator fell back to sys.executable (uv's
   standalone Python) when running under 'uv run', because
   sys.prefix == sys.base_prefix (uv doesn't set up traditional
   venv activation). The generated ExecStart pointed to a Python
   binary without site-packages, crashing the service on startup.

   Fix: check VIRTUAL_ENV env var before falling back to
   sys.executable. uv sets VIRTUAL_ENV even when sys.prefix
   doesn't reflect the venv.

3. Nudge model to continue after empty post-tool response (#9400)
   Weaker models (GLM-5, mimo-v2-pro) sometimes return empty
   responses after tool calls instead of continuing to the next
   step. The agent silently abandoned the remaining work with
   '(empty)' or used prior-turn fallback text.

   Fix: when the model returns empty after tool calls AND there's
   no prior-turn content to fall back on, inject a one-time user
   nudge message telling the model to process the tool results and
   continue. The flag resets after each successful tool round so it
   can fire again on later rounds.

Test plan: 97 gateway + CLI tests pass, 9 venv detection tests pass

hermes_cli/gateway.py

@@ -715,7 +715,9 @@ def _detect_venv_dir() -> Path | None:
     """Detect the active virtualenv directory.
 
     Checks ``sys.prefix`` first (works regardless of the directory name),
-    then falls back to probing common directory names under PROJECT_ROOT.
+    then ``VIRTUAL_ENV`` env var (covers uv-managed environments where
+    sys.prefix == sys.base_prefix), then falls back to probing common
+    directory names under PROJECT_ROOT.
     Returns ``None`` when no virtualenv can be found.
     """
     # If we're running inside a virtualenv, sys.prefix points to it.
@@ -724,6 +726,15 @@ def _detect_venv_dir() -> Path | None:
         if venv.is_dir():
             return venv
 
+    # uv and some other tools set VIRTUAL_ENV without changing sys.prefix.
+    # This catches `uv run` where sys.prefix == sys.base_prefix but the
+    # environment IS a venv.  (#8620)
+    _virtual_env = os.environ.get("VIRTUAL_ENV")
+    if _virtual_env:
+        venv = Path(_virtual_env)
+        if venv.is_dir():
+            return venv
+
     # Fallback: check common virtualenv directory names under the project root.
     for candidate in (".venv", "venv"):
         venv = PROJECT_ROOT / candidate