platform/webui: add scenario description, show it in the Scenario page's list

ScenarioSpec gains an optional "description" field. Listing (both
ListLocalScenarioFiles and ListScenarioFilesOnRemovableMedia, via the new
scenarioDescription helper) reads it out of each file without requiring
full ParseScenarioJSON validation to succeed, so a listing never hides a
scenario over an unrelated validation issue. The webui Scenario page now
renders Name/Description/Found-on/Run instead of just Name/Found-on — a
bare filename rarely tells anyone but the author what a scenario actually
does.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/platform/scenario.go

@@ -45,6 +45,7 @@ type ScenarioJob struct {
 //
 //	{
 //	  "name": "nvbandwidth-all-gpu-power-watch",
+//	  "description": "Full nvbandwidth across all GPUs at once — the failure mode never reproduces on a single socket alone — while sampling IPMI sensors and GPU power/temp for a power-delivery correlation.",
 //	  "timeout_sec": 1800,
 //	  "jobs": [
 //	    {"name": "ipmi-sensors", "type": "sampler", "interval_sec": 2,
@@ -57,9 +58,15 @@ type ScenarioJob struct {
 //	  ]
 //	}
 type ScenarioSpec struct {
-	Name       string        `json:"name"`
-	TimeoutSec int           `json:"timeout_sec,omitempty"`
-	Jobs       []ScenarioJob `json:"jobs"`
+	Name string `json:"name"`
+	// Description is a short, human-readable explanation of what the
+	// scenario does and why — shown in the webui's scenario list (and
+	// available to any other UI) alongside the name, since a bare
+	// filename/name rarely conveys enough for someone other than the
+	// author to decide whether to run it.
+	Description string        `json:"description,omitempty"`
+	TimeoutSec  int           `json:"timeout_sec,omitempty"`
+	Jobs        []ScenarioJob `json:"jobs"`
 }
 
 // ParseScenarioJSON parses and validates a scenario file's contents.