feat(skills): add Cursor plugin manifest

Closes #662. The Codex/Cursor/opencode plugin PR (#507) added Cursor as a
`pup skills install` platform and shipped Codex marketplace manifests, but
never created the Cursor plugin manifest. Cursor discovers plugins via
`.cursor-plugin/plugin.json` (mirror of `.claude-plugin/`), so without it
Cursor Cloud Agents can't pre-install the pup skills from the marketplace.

- Add .cursor-plugin/plugin.json mirroring the .codex-plugin sibling
  (name, version, description, author, repository, license, keywords,
  and skills => ./skills/).
- Document the Cursor manifest alongside Claude Code and Codex in README.

Author: datadog-bits

.cursor-plugin/plugin.json

@@ -0,0 +1,13 @@
+{
+  "name": "pup",
+  "version": "0.62.0",
+  "description": "Datadog API CLI with 49 command groups, 300+ subcommands. Skills and domain agents for monitoring, logs, APM, security, and infrastructure.",
+  "author": {
+    "name": "Datadog",
+    "email": "support@datadoghq.com"
+  },
+  "repository": "https://github.com/DataDog/pup",
+  "license": "Apache-2.0",
+  "keywords": ["datadog", "monitoring", "logs", "apm", "metrics", "security", "infrastructure"],
+  "skills": "./skills/"
+}

README.md

@@ -599,6 +599,8 @@ Pup ships plugin manifest files for several AI coding assistants:
 /plugin marketplace add DataDog/pup
 
 # Codex (reads .codex-plugin/plugin.json from the repo, or marketplace.json from ~/.agents/plugins/)
+
+# Cursor (reads .cursor-plugin/plugin.json from the repo)
 ```
 
 ## ACP Server