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

plugins/core-essentials/skills/delegate-agy/SKILL.md

@@ -1,3 +1,11 @@
+---
+name: delegate-agy
+description: Delegates a subtask to the Antigravity CLI (agy) when the user wants a second opinion, external grounded research, or needs a large multi-file audit (>500 lines) processed without bloating the current context.
+version: 1.0.0
+user-invocable: true
+author: Rootiest
+---
+
 # Antigravity Subagent Delegation (`delegate-agy`)
 
 When the user asks for a second opinion, external grounded research, or when a task requires processing large multi-file audits (>500 lines) that would bloat context, delegate the subtask to `agy`.