feat: restructure into a multi-extension plugin framework for Claude Code and agy

Replace the skills-only SSoT (skills/<name>/SKILL.md) with plugins/<name>/,
where a plugin can bundle skills, MCP servers, lifecycle hooks, rules,
commands, and agents. scripts/generate_plugins.py now discovers plugins
from one or more layered --source roots and emits real per-target trees
(dist/claude-code/** + .claude-plugin/marketplace.json, dist/agy/**),
translating hooks.json/mcp.json between Claude Code's and agy's actual
schemas instead of only fanning out flat skill lists.

Adds a private-overlay build path (--source/--private-repo/--out/
--install-local) so a second, non-public repo (PII, tokens, personal-only
plugins) can extend or override the public plugin set without either repo
touching the other's history — private builds default to a gitignored
dist-private/ and never land in tracked output.

Drops install.sh and descriptions.json: both only ever covered skills and
pre-date the native plugin marketplace; both tools now install the plugin
marketplace the documented way (Claude Code's /plugin marketplace add,
agy's plugins.json entries). CI path filters move to plugins/**, and the
workflow gains workflow_dispatch for a manual re-trigger.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014YVWWAnvR5TDQzmfY3RXG9

.claude-plugin/marketplace.json

@@ -6,69 +6,41 @@
   },
   "description": "Reusable AI skills for Claude Code and Antigravity CLI",
   "plugins": [
+    {
+      "name": "core-essentials",
+      "source": "./dist/claude-code/core-essentials"
+    },
     {
       "name": "date-time",
-      "description": "Retrieves the exact current date and time for time-sensitive queries, scheduling, duration calculations, and validating chronological context.",
-      "source": "./",
-      "skills": [
-        "./skills/date-time"
-      ]
+      "source": "./dist/claude-code/date-time"
     },
     {
       "name": "docs-sync-audit",
-      "description": "Analyzes repository delta since the last documentation update and synchronizes the Single Source of Truth (docs/, wiki, or README) with the current codebase state.",
-      "source": "./",
-      "skills": [
-        "./skills/docs-sync-audit"
-      ]
+      "source": "./dist/claude-code/docs-sync-audit"
     },
     {
       "name": "git-publish-workflow",
-      "description": "Automates branching, conventional commits, testing, and PR creation for uncommitted or staged work. Handles both independent and stacked PRs.",
-      "source": "./",
-      "skills": [
-        "./skills/git-publish-workflow"
-      ]
+      "source": "./dist/claude-code/git-publish-workflow"
     },
     {
       "name": "readme-sync-audit",
-      "description": "Analyzes repository delta since the last documentation update and synchronizes the README.md with the current codebase state.",
-      "source": "./",
-      "skills": [
-        "./skills/readme-sync-audit"
-      ]
+      "source": "./dist/claude-code/readme-sync-audit"
     },
     {
       "name": "ship-it",
-      "description": "Runs a comprehensive pre-flight audit, syncs the README, and publishes the changes to a new PR.",
-      "source": "./",
-      "skills": [
-        "./skills/ship-it"
-      ]
+      "source": "./dist/claude-code/ship-it"
     },
     {
       "name": "systematic-enumeration",
-      "description": "Forces element-by-element verification for finite sets to prevent counting errors.",
-      "source": "./",
-      "skills": [
-        "./skills/systematic-enumeration"
-      ]
+      "source": "./dist/claude-code/systematic-enumeration"
     },
     {
       "name": "technical-devlog-scribe",
-      "description": "Generates a highly structured, objective technical summary of a development session.",
-      "source": "./",
-      "skills": [
-        "./skills/technical-devlog-scribe"
-      ]
+      "source": "./dist/claude-code/technical-devlog-scribe"
     },
     {
       "name": "rootiest-ai-all",
-      "description": "Reusable AI skills for Claude Code and Antigravity CLI (all skills)",
-      "source": "./",
-      "skills": [
-        "./skills/"
-      ]
+      "source": "./dist/claude-code/rootiest-ai-all"
     }
   ]
 }