feat: native plugin-install for Codex/Cursor/Gemini + CLAUDE.md→AGENTS.md symlink

Add lean, native plugin-install entry points so each harness's own plugin
manager can install this marketplace (mirroring obra/superpowers) — committing
only small JSON registries, not duplicated skill/agent content trees.

- Codex: committed marketplace registry (.agents/plugins/marketplace.json) +
  per-plugin manifests (plugins/*/.codex-plugin/plugin.json). Entries point at
  source ./plugins/<name>; Codex reads SKILL.md directly. Transformed
  .codex/skills|agents stay gitignored.
- Cursor: commit the existing .cursor-plugin/ marketplace + .cursor/rules/
  (these already point at source plugins/).
- Gemini: gemini-extension.json already committed (contextFileName: AGENTS.md);
  transformed trees stay gitignored (install via clone + make generate).
- OpenCode: unchanged — install via `make install-opencode` (transformed tree
  stays gitignored).
- CLAUDE.md is now a symlink to AGENTS.md; Claude-Code addenda moved to
  docs/harnesses.md.
- CI: new step fails if `make generate-all` drifts from the committed registries.

Net new committed: ~720 KB of manifests (no skill/agent duplication). Adds
round-trip tests for the new registries + the symlink. Docs updated across
README, docs/harnesses.md, ARCHITECTURE.md, CONTRIBUTING.md, GEMINI.md,
docs/authoring.md, and the PR template.

plugins/javascript-typescript/.codex-plugin/plugin.json

@@ -0,0 +1,16 @@
+{
+  "name": "javascript-typescript",
+  "version": "1.2.3",
+  "description": "JavaScript and TypeScript development with ES6+, Node.js, React, and modern web frameworks",
+  "skills": "./skills/",
+  "author": {
+    "name": "Seth Hobson",
+    "email": "seth@major7apps.com"
+  },
+  "license": "MIT",
+  "interface": {
+    "displayName": "Javascript Typescript",
+    "shortDescription": "JavaScript and TypeScript development with ES6+, Node.js, React, and modern web frameworks",
+    "category": "Coding"
+  }
+}