feat(#766): distribute gsd-core as a native Claude Code plugin (#797)

* feat(#766): distribute gsd-core as a native Claude Code plugin

Add an additive .claude-plugin/plugin.json manifest plus hooks/hooks.json so gsd-core can be installed as a first-class Claude Code plugin (marketplace or zero-friction @skills-dir), with /gsd-core: namespaced commands and lifecycle management — alongside the unchanged npm/file-copy installer.

- .claude-plugin/plugin.json: validated with 'claude plugin validate --strict'
- hooks/hooks.json: mirrors the installer's always-on Claude hook wiring via ${CLAUDE_PLUGIN_ROOT}
- package.json: ship .claude-plugin in the npm tarball
- tests/issue-766-plugin-manifest.test.cjs: manifest + always-on-hook-contract drift guards
- docs: install-on-your-runtime.md + FEATURES.md

Closes #766

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(#766): add ADR-766 + glossary entry for Claude Code Plugin Manifest Module

Record the plugin manifest as the Seam projecting gsd-core's artifact surfaces onto the Claude Code plugin contract (sibling of the Runtime Artifact Layout Module, ADR-3660), with the defined kind->field mapping and the always-on hook projection rule.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -0,0 +1,16 @@
+{
+  "name": "gsd-core",
+  "displayName": "GSD Core",
+  "version": "1.3.1-dev.0",
+  "description": "GSD Core is a meta-prompting, context engineering, and spec-driven development system for AI coding agents.",
+  "author": {
+    "name": "open-gsd",
+    "url": "https://github.com/open-gsd"
+  },
+  "homepage": "https://github.com/open-gsd/gsd-core",
+  "repository": "https://github.com/open-gsd/gsd-core",
+  "license": "MIT",
+  "keywords": ["spec-driven-development", "planning", "workflow", "context-engineering", "claude-code", "gsd"],
+  "commands": "./commands/gsd/",
+  "hooks": "./hooks/hooks.json"
+}