fix(impl-generate): preserve created across regens + correct model string

Two metadata bugs fixed at the source:

1. `created` was overwritten with the current timestamp on every regen,
   silently erasing the first-generation date. Now the generator reads the
   existing metadata file (if present) and preserves its `created` value;
   only `updated` advances. The impl-review step already uses
   `created != updated` to decide whether to print "Created: ..." or
   "Updated: ..." in the implementation header — that logic only works
   when `created` is immutable.

2. `generated_by: claude-opus-4-5-20251101` was frozen from the era when
   the workflow was written. The Claude action actually runs
   `--model opus`, which resolves to whatever the current opus alias is
   (today Opus 4.7). Switching to the family name `claude-opus` is honest
   + future-proof, and stops the metadata from lying every model release.

The scatter-basic pilot already landed 9 files with today's date as
`created`. Those will stabilise on the next regen (this fix) or can be
patched in place if needed.

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

Author: MarkusNeusinger

.github/workflows/impl-generate.yml

@@ -391,7 +391,7 @@ jobs:
           # Write metadata file using Python for proper YAML formatting
           # Pass all variables inline to avoid export/env issues
           .venv/bin/python3 -c "
-          import yaml
+          import os, yaml
           lib = '$LIBRARY'
           spec = '$SPEC_ID'
           language = '$LANGUAGE'
@@ -403,13 +403,31 @@ jobs:
           has_html = '$HAS_HTML' == 'true'
           metadata_file = '$METADATA_FILE'
           base_url = f'https://storage.googleapis.com/anyplot-images/plots/{spec}/{language}/{lib}'
+
+          # Preserve the original 'created' timestamp on regenerations.
+          # 'created' is the first-generation date — it must never be overwritten.
+          # Only 'updated' moves forward on every regen.
+          created_ts = ts
+          if os.path.exists(metadata_file):
+              try:
+                  with open(metadata_file) as f:
+                      existing = yaml.safe_load(f) or {}
+                  if existing.get('created'):
+                      created_ts = existing['created']
+              except Exception:
+                  pass
+
           data = {
               'library': lib,
               'language': language,
               'specification_id': spec,
-              'created': ts,
+              'created': created_ts,
               'updated': ts,
-              'generated_by': 'claude-opus-4-5-20251101',
+              # Reflects what claude_args=`--model opus` actually runs: whatever
+              # Claude Code's current "opus" alias resolves to (today Opus 4.7).
+              # Use the family name instead of a frozen version string so the
+              # metadata doesn't go stale every model release.
+              'generated_by': 'claude-opus',
               'workflow_run': run_id,
               'issue': issue,
               'python_version': py_ver,