feat(mcp-server): scheduler_pend_reasons tool — bjobs -p equivalent

New read-only tool that answers "why is my job waiting": lists PEND jobs (or
one by id) and translates each job's pending_reason into a human-readable
scenario (LSF bjobs -p style). Maps the PREASON_* codes surfaced by the
PEND-reason work (4e2ac86): no_resource / match_fail / limit_global /
limit_host / queue_closed / queue_draining / time_window / host_unavail.

- internal/pendreason.go: pending_reason JSON parsing (mirrors apiserver
  rawPendReason) + reasonScenario() translation + bjobs -p style summary.
- internal/app.go: HandlePendReasons — id param for one job, else pending
  list filtered by queue/owner/limit.
- cmd/main.go: register scheduler_pend_reasons (read-only).
- 5 unit tests (known/unknown codes, parse from JSON, missing reason,
  summary format). go test ./... + go vet clean.

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

src/frontend/mcp-server/cmd/main.go

@@ -159,6 +159,17 @@ func main() {
 		mcp.WithDestructiveHintAnnotation(false),
 	), app.HandleEstimateWaitTime)
 
+	// ── scheduler_pend_reasons (bjobs -p equivalent) ──
+	s.AddTool(mcp.NewTool("scheduler_pend_reasons",
+		mcp.WithDescription("Explain why PEND (queued) jobs are waiting — the bjobs -p equivalent. Lists pending jobs (or one by id) and translates each job's PEND reason into a human-readable scenario (no_resource, match_fail, limit_host/global, queue_closed, host_unavail, ...)."),
+		mcp.WithString("id", mcp.Description("Specific job id to explain (optional; defaults to all pending jobs)")),
+		mcp.WithString("queue", mcp.Description("Filter by queue name")),
+		mcp.WithString("owner", mcp.Description("Filter by job owner username")),
+		mcp.WithNumber("limit", mcp.Description("Max results to return")),
+		mcp.WithReadOnlyHintAnnotation(true),
+		mcp.WithDestructiveHintAnnotation(false),
+	), app.HandlePendReasons)
+
 	// ── write tools (Phase 1) ──
 	// Write tools are annotated destructive so the MCP client (e.g. Claude
 	// Code) prompts the user for confirmation before executing them — the