fix(bmad-prd): normalize status casing and add friendly file errors

- compute_stats: lower-case `status` before bucketing so findings with
  any casing (e.g. "Pass") feed the stat buckets and the score bar
  fills correctly. Matches the .lower() pattern already used in
  render_finding and render_finding_md.
- main: wrap findings/template read_text calls; emit a one-line error
  to stderr and return 1 on FileNotFoundError or JSONDecodeError
  instead of dumping a raw traceback. Script is LLM-invoked, so a
  clean diagnostic is the contract.

Addresses augmentcode review comments 3235100013 and 3235100018.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/bmm-skills/2-plan-workflows/bmad-prd/scripts/render-validation-html.py

@@ -46,7 +46,7 @@ def compute_stats(findings: list[dict]) -> dict:
     failed_critical = 0
     failed_high = 0
     for f in findings:
-        status = f.get("status", "n/a")
+        status = (f.get("status") or "n/a").lower()
         if status in by_status:
             by_status[status] += 1
         if status == "fail":
@@ -219,8 +219,19 @@ def main(argv: list[str]) -> int:
     template_path = Path(args.template)
     output_path = Path(args.output)
 
-    data = json.loads(findings_path.read_text())
-    template = template_path.read_text()
+    try:
+        data = json.loads(findings_path.read_text())
+    except FileNotFoundError:
+        print(f"error: findings file not found: {findings_path}", file=sys.stderr)
+        return 1
+    except json.JSONDecodeError as e:
+        print(f"error: findings file is not valid JSON ({findings_path}): {e}", file=sys.stderr)
+        return 1
+    try:
+        template = template_path.read_text()
+    except FileNotFoundError:
+        print(f"error: template file not found: {template_path}", file=sys.stderr)
+        return 1
 
     findings = data.get("findings", []) or []