fix(scripts): skip the portal probe on niri in inject-probe

The injector never offers the RemoteDesktop portal on niri, but inject-probe
ran the portal probe unguarded and reported portal_available: true there.
The probe now reuses the injector's _is_niri_session guard: on niri it skips
the portal and reports portal_skip_reason "niri" with portal_available false.
Off niri the probe runs and reports its answer as before.

Fixes #46

docs/PERF_BASELINE_PROTOCOL.md

@@ -63,10 +63,12 @@ with the installed build's interpreter.
   version-probe result and timing, then a session-establishment attempt
   (fast failure versus hang matters more than the version answer), plus
   the resolved first rung and a no-emission `wtype ""` spawn cost as a
-  lower bound. End-to-end key-to-screen latency is deliberately not
-  taken here: emitting keystrokes into a live desktop from a background
-  run would disturb the session. It belongs to the hardware checklist
-  with a dedicated target window.
+  lower bound. On niri the portal probe is skipped, as the injector skips
+  it, and the report says so with `portal_skip_reason: "niri"` and
+  `portal_available: false`. End-to-end key-to-screen latency is
+  deliberately not taken here: emitting keystrokes into a live desktop
+  from a background run would disturb the session. It belongs to the
+  hardware checklist with a dedicated target window.
 - **Thresholds for the model ladder:** derive free-memory cutoffs from
   the maximum observed per-rung footprint (transcribe peak, not just
   load) plus explicit headroom for the compositor and other GPU