Inject update instructions into SKILL.md at zip-packaging time

Appends a standard "Updating this skill" section to SKILL.md inside each
zip (both individual and fat), pointing agents to .version for the registry
URL and commit to compare. Source files are not modified.

This lets an agent with the skill installed know how to check for and apply
updates on a cold start, without any prior context.

Author: avillar

.github/scripts/build_site.py

@@ -50,6 +50,15 @@ def parse_frontmatter(path: Path) -> tuple[str, str]:
     return name, desc
 
 
+UPDATE_SECTION = """
+## Updating this skill
+
+This skill was installed from the OGC LLM Skills registry. To check for
+updates, read `.version` in this directory — it contains `commit`, `date`,
+`zip_url`, and `llms_txt`. Compare `commit` against the registry manifest
+and follow the update instructions at the `llms_txt` URL.
+"""
+
 skills = []
 repo_root = Path(".")
 # Collect (skill_dir, zip_stem, files) for the fat zip
@@ -74,21 +83,30 @@ def parse_frontmatter(path: Path) -> tuple[str, str]:
 
     with zipfile.ZipFile(zip_path, "w", zipfile.ZIP_DEFLATED) as zf:
         for f in skill_files:
-            zf.write(f, f.relative_to(skill_dir))
+            if f == skill_md:
+                zf.writestr(str(f.relative_to(skill_dir)),
+                            f.read_text(encoding="utf-8") + UPDATE_SECTION)
+            else:
+                zf.write(f, f.relative_to(skill_dir))
         zf.writestr(".version", json.dumps(version_obj, indent=2))
 
     name, desc = parse_frontmatter(skill_md)
     skills.append({"name": name, "description": desc, "zip": zip_name,
                    "commit": commit, "date": date})
-    all_skill_entries.append((zip_stem, skill_dir, skill_files, version_obj))
+    all_skill_entries.append((zip_stem, skill_dir, skill_md, skill_files, version_obj))
     print(f"  packaged: {zip_name}  ({name})")
 
 # Fat zip: all skills with their zip-stem as the directory prefix
 fat_zip_name = "all-skills.zip"
 with zipfile.ZipFile(out_dir / fat_zip_name, "w", zipfile.ZIP_DEFLATED) as zf:
-    for zip_stem, skill_dir, skill_files, version_obj in all_skill_entries:
+    for zip_stem, skill_dir, skill_md, skill_files, version_obj in all_skill_entries:
         for f in skill_files:
-            zf.write(f, Path(zip_stem) / f.relative_to(skill_dir))
+            arc_path = Path(zip_stem) / f.relative_to(skill_dir)
+            if f == skill_md:
+                zf.writestr(str(arc_path),
+                            f.read_text(encoding="utf-8") + UPDATE_SECTION)
+            else:
+                zf.write(f, arc_path)
         zf.writestr(str(Path(zip_stem) / ".version"), json.dumps(version_obj, indent=2))
 print(f"  packaged: {fat_zip_name}  (all skills)")