feat(plugin): automatic updates via native marketplaces and a version gate

Installs previously drifted: a standalone `npx skills add` copy never
followed the repo, and Claude Code disables auto-update for third-party
marketplaces by default. Route every client through its own marketplace so
merged changes reach users through the native update path.

- Add `.agents/plugins/marketplace.json` so Codex resolves the plugin from
  a Git marketplace it refreshes at startup.
- Add `author` to the Claude manifest so both native manifests agree.
- Add `scripts/bump-plugin-version.py` to increment the Claude and Codex
  versions together; it refuses to run when they already differ.
- Add `scripts/verify-plugin-package.py` to gate synchronized, strictly
  increasing semver against the base ref, marketplace paths that stay
  inside the repo root, and the packaged skill.
- Add `scripts/test-plugin-package.py` covering the adversarial cases:
  missing bump, mismatched manifests, malformed semver, and a marketplace
  target that does not exist.
- Add a `plugin-package` CI job running the tests, the version gate against
  the PR base sha, and `claude plugin validate . --strict`.
- Document the per-PR bump requirement, the new gates, and the per-client
  install/update paths, including the one-time migration off standalone
  copies.

Version bumps 2.3.0 -> 2.3.1 to satisfy the new gate.

.claude-plugin/plugin.json

@@ -1,7 +1,11 @@
 {
   "name": "diagram-design",
   "description": "Create branded architecture, IT current-state, flowchart, sequence, state machine, ER/data model, timeline, swimlane, quadrant, radar/spider, loop/flywheel, nested, tree, org chart, layer stack, Venn, pyramid/funnel, bar, line, Gantt and scatter charts, high-level, process, medallion, data flow, DP integration, or DP security matrix diagrams as standalone HTML/SVG/PNG. Redraw .drawio/.drawio.png/.drawio.svg or Mermaid .mmd sources at a chosen size/detail; onboard brand tokens from a website; add semantic patterns, callouts, accessible motion, or sketchy/hand-drawn styling.",
-  "version": "2.3.0",
+  "version": "2.3.1",
+  "author": {
+    "name": "Cathryn Lavery",
+    "url": "https://github.com/cathrynlavery"
+  },
   "homepage": "https://github.com/cathrynlavery/diagram-design",
   "repository": "https://github.com/cathrynlavery/diagram-design",
   "license": "MIT",