Make discover count machine-readable for CI parsing

The workflow's grep-on-free-text approach was fragile: local run correctly
extracted 162, but in the CI run the 'Open issue' step was skipped, meaning
new_count ended up as 0 after parsing. Likely causes: stdout buffering,
color codes, or shell differences between local and the Gitea runner's
shell.

Fix: tools/refresh_sku_catalog.py --discover now emits a deterministic
last-line marker `DISCOVER_MISSING_COUNT=<n>`. The workflow parses that
with sed (anchored, unambiguous) instead of the human-facing summary
line. Also added a diagnostic echo so the parsed value shows up in CI
logs for future debugging.

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

tools/refresh_sku_catalog.py

@@ -385,6 +385,8 @@ def discover_catalog(verbose=False):
 
     missing = discover_missing_skus(api_map, sku_map, verbose=verbose)
     print_missing_skus(missing, limit=None if verbose else 40)
+    # Machine-readable summary for CI/automation — always last line on stdout.
+    print("\nDISCOVER_MISSING_COUNT={}".format(len(missing)))
     return 0