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/agentskills-skill.schema.json

@@ -0,0 +1,46 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://agentskills.io/schema/skill-frontmatter.json",
+  "title": "agentskills.io SKILL.md frontmatter",
+  "description": "Strict JSON Schema for the YAML frontmatter of a SKILL.md file per the agentskills.io open standard (Anthropic, 2025-12-18). Only `name` and `description` are required; a small optional set is permitted; every other top-level key is a non-standard 'unexpected field'. Constraints that cannot be expressed in JSON Schema (name must equal the parent directory; frontmatter must contain no angle brackets) are enforced by tools/validate-agentskills.py.",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["name", "description"],
+  "properties": {
+    "name": {
+      "type": "string",
+      "minLength": 1,
+      "maxLength": 64,
+      "pattern": "^[a-z0-9]+(-[a-z0-9]+)*$",
+      "description": "1-64 chars, lowercase alphanumeric and single hyphens only; no leading/trailing/consecutive hyphens; must match the parent directory name; must not be a reserved word."
+    },
+    "description": {
+      "type": "string",
+      "minLength": 1,
+      "maxLength": 1024,
+      "description": "1-1024 chars. Must state BOTH what the skill does and when to use it."
+    },
+    "license": {
+      "type": "string",
+      "minLength": 1,
+      "description": "SPDX license identifier or license text."
+    },
+    "compatibility": {
+      "type": "string",
+      "maxLength": 500,
+      "description": "Free-text compatibility notes (<=500 chars)."
+    },
+    "metadata": {
+      "type": "object",
+      "description": "Arbitrary key/value map for non-standard fields. This is where domain, subdomain, tags, version, author, and framework mappings (mitre_attack, nist_csf, atlas_techniques, d3fend_techniques, nist_ai_rmf, mitre_f3) belong under strict compliance.",
+      "additionalProperties": true
+    },
+    "allowed-tools": {
+      "description": "EXPERIMENTAL. Tools the skill is permitted to use; string or array of strings.",
+      "oneOf": [
+        { "type": "string" },
+        { "type": "array", "items": { "type": "string" } }
+      ]
+    }
+  }
+}