exporter: surface a skipped module's warning as its placeholder reason

unavailable() only ever read module.errors for the placeholder detail
text, so a deliberately skipped section (speed in --quick or target
mode, which reports its reason as a warning, not an error) always
fell through to the generic 'no data was collected for this section'
line even though the real reason ('speedtest skipped in target mode')
was sitting right there in the json. spec says a skipped speedtest
must say why, not just that it's missing.

src/netcheck/exporter.py

@@ -208,6 +208,8 @@ def unavailable(report: dict[str, Any], section: str) -> str | None:
     if module.get("status") in ("ok", "partial") and module.get("data") is not None:
         return None
     detail = "; ".join(f"{e.get('source')}: {e.get('message')}" for e in module.get("errors") or [])
+    if not detail:
+        detail = "; ".join(module.get("warnings") or [])
     if not detail:
         detail = "no data was collected for this section"
     return f"_Not available — {module.get('status', 'skipped')}: {detail}._"

tests/test_exporter.py

@@ -570,6 +570,18 @@ def test_unavailable_explains_a_skipped_module_without_inventing_an_error():
     assert "skipped" in note
 
 
+def test_unavailable_surfaces_the_skip_reason_from_warnings():
+    report = build_report(
+        meta(),
+        {"speed": ModuleResult(name="speed", status="skipped", warnings=["speedtest skipped in target mode"])},
+        [],
+        {},
+    )
+    note = unavailable(report, "speed")
+    assert note is not None
+    assert "speedtest skipped in target mode" in note
+
+
 def test_an_all_modules_failed_run_still_renders_every_section():
     text = render_markdown(dead_report())
     headings = [line for line in text.splitlines() if line.startswith("## ")]