Add agentskills.io compliance schema and validator

- tools/agentskills-skill.schema.json: strict JSON Schema for the
  agentskills.io SKILL.md frontmatter standard (name+description required;
  optional license/compatibility/metadata/allowed-tools; no other top-level
  keys).
- tools/validate-agentskills.py: read-only compliance validator (also checks
  name==directory and the no-angle-brackets rule).

tools/validate-agentskills.py

@@ -0,0 +1,137 @@
+#!/usr/bin/env python3
+"""Validate SKILL.md frontmatter against the strict agentskills.io standard.
+
+Reports, per skill, any deviation from tools/agentskills-skill.schema.json plus
+the two constraints JSON Schema can't express (name == parent dir; no angle
+brackets in frontmatter). READ-ONLY; never edits files.
+
+Usage:
+  python3 tools/validate-agentskills.py            # summary + report
+  python3 tools/validate-agentskills.py --json      # machine-readable JSON
+  python3 tools/validate-agentskills.py --strict    # exit 1 if any non-compliant
+"""
+import os, re, sys, json, glob
+from collections import Counter
+
+REPO = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+ALLOWED = {"name", "description", "license", "compatibility", "metadata", "allowed-tools"}
+NAME_RE = re.compile(r"^[a-z0-9]+(-[a-z0-9]+)*$")
+
+def top_level_keys_and_scalars(fm):
+    """Minimal YAML: top-level keys (col 0) + scalar values for name/description."""
+    keys = []
+    scalars = {}
+    lines = fm.split("\n")
+    for i, line in enumerate(lines):
+        m = re.match(r"^([A-Za-z0-9_-]+):(.*)$", line)
+        if not m:
+            continue
+        key, rest = m.group(1), m.group(2)
+        keys.append(key)
+        val = rest.strip()
+        if val and val[0] in "|>":  # block scalar -> gather following indented lines
+            buf = []
+            for nxt in lines[i + 1:]:
+                if re.match(r"^\s+\S", nxt):
+                    buf.append(nxt.strip())
+                elif nxt.strip() == "":
+                    buf.append("")
+                else:
+                    break
+            val = " ".join(x for x in buf if x != "").strip()
+        elif not val:
+            # could be a folded plain scalar wrapped onto continuation lines
+            buf = []
+            for nxt in lines[i + 1:]:
+                if re.match(r"^\s+-\s", nxt) or re.match(r"^[A-Za-z0-9_-]+:", nxt):
+                    break
+                if re.match(r"^\s+\S", nxt):
+                    buf.append(nxt.strip())
+                else:
+                    break
+            val = " ".join(buf).strip()
+        scalars[key] = val.strip().strip("\"'")
+    return keys, scalars
+
+def validate(path):
+    slug = os.path.basename(os.path.dirname(path))
+    text = open(path, encoding="utf-8").read()
+    m = re.match(r"^---\n(.*?)\n---", text, re.DOTALL)
+    problems = []
+    if not m:
+        return slug, ["no YAML frontmatter block"], []
+    fm = m.group(1)
+    keys, scalars = top_level_keys_and_scalars(fm)
+
+    if "name" not in keys:
+        problems.append("missing required key: name")
+    if "description" not in keys:
+        problems.append("missing required key: description")
+
+    name = scalars.get("name", "")
+    if name:
+        if not NAME_RE.match(name):
+            problems.append(f"name not lowercase-kebab-case: {name!r}")
+        if not (1 <= len(name) <= 64):
+            problems.append(f"name length {len(name)} out of 1..64")
+        if name != slug:
+            problems.append(f"name {name!r} != directory {slug!r}")
+
+    desc = scalars.get("description", "")
+    if desc:
+        if not (1 <= len(desc) <= 1024):
+            problems.append(f"description length {len(desc)} out of 1..1024")
+    elif "description" in keys:
+        problems.append("description empty")
+
+    if "<" in fm or ">" in fm:
+        problems.append("frontmatter contains angle brackets (injection risk / not allowed)")
+
+    nonstd = [k for k in keys if k not in ALLOWED]
+    for k in nonstd:
+        problems.append(f"non-standard top-level key: {k}")
+    return slug, problems, nonstd
+
+def main():
+    as_json = "--json" in sys.argv
+    strict = "--strict" in sys.argv
+    skills = sorted(glob.glob(os.path.join(REPO, "skills", "*", "SKILL.md")))
+    results = []
+    nonstd_hist = Counter()
+    compliant = 0
+    for p in skills:
+        slug, problems, nonstd = validate(p)
+        nonstd_hist.update(nonstd)
+        if not problems:
+            compliant += 1
+        results.append({"skill": slug, "compliant": not problems, "problems": problems})
+    noncompliant = [r for r in results if not r["compliant"]]
+    summary = {
+        "total": len(skills),
+        "compliant": compliant,
+        "noncompliant": len(noncompliant),
+        "nonstandard_key_frequency": dict(nonstd_hist.most_common()),
+    }
+    if as_json:
+        print(json.dumps({"summary": summary, "results": results}, indent=1))
+    else:
+        print(f"agentskills.io compliance: {compliant}/{len(skills)} compliant, "
+              f"{len(noncompliant)} non-compliant")
+        print("\nNon-standard top-level keys (count of skills carrying each):")
+        for k, n in nonstd_hist.most_common():
+            print(f"  {k:20s} {n}")
+        # distinct problem types (excluding the per-key nonstd noise)
+        other = Counter()
+        for r in noncompliant:
+            for pr in r["problems"]:
+                if not pr.startswith("non-standard top-level key:"):
+                    other[re.sub(r':.*$', '', pr)] += 1
+        if other:
+            print("\nOther (non-key) issues:")
+            for k, n in other.most_common():
+                print(f"  {k}: {n}")
+    if strict and noncompliant:
+        sys.exit(1)
+
+if __name__ == "__main__":
+    main()