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

dist/agy/core-essentials/plugin.json

@@ -0,0 +1,3 @@
+{
+  "name": "core-essentials"
+}

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

@@ -0,0 +1,18 @@
+---
+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`.
+
+## Execution Syntax
+Run `agy` in headless, non-interactive mode using the bash tool:
+
+```bash
+agy --dangerously-skip-permissions -p "<detailed_task_prompt>"
+```

dist/agy/date-time/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "date-time",
-  "description": "Retrieves the exact current date and time for time-sensitive queries, scheduling, duration calculations, and validating chronological context."
+  "name": "date-time"
 }

dist/agy/docs-sync-audit/plugin.json

@@ -1,4 +1,3 @@
 {
-  "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."
+  "name": "docs-sync-audit"
 }

dist/agy/git-publish-workflow/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "git-publish-workflow",
-  "description": "Automates branching, conventional commits, testing, and PR creation for uncommitted or staged work. Handles both independent and stacked PRs."
+  "name": "git-publish-workflow"
 }

dist/agy/readme-sync-audit/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "readme-sync-audit",
-  "description": "Analyzes repository delta since the last documentation update and synchronizes the README.md with the current codebase state."
+  "name": "readme-sync-audit"
 }

dist/agy/rootiest-ai-all/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "rootiest-ai-all",
-  "description": "Reusable AI skills for Claude Code and Antigravity CLI (all skills)"
+  "name": "rootiest-ai-all"
 }

dist/agy/rootiest-ai-all/skills/delegate-agy/SKILL.md

@@ -0,0 +1,18 @@
+---
+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`.
+
+## Execution Syntax
+Run `agy` in headless, non-interactive mode using the bash tool:
+
+```bash
+agy --dangerously-skip-permissions -p "<detailed_task_prompt>"
+```

dist/agy/ship-it/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "ship-it",
-  "description": "Runs a comprehensive pre-flight audit, syncs the README, and publishes the changes to a new PR."
+  "name": "ship-it"
 }

dist/agy/systematic-enumeration/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "systematic-enumeration",
-  "description": "Forces element-by-element verification for finite sets to prevent counting errors."
+  "name": "systematic-enumeration"
 }

dist/agy/technical-devlog-scribe/plugin.json

@@ -1,4 +1,3 @@
 {
-  "name": "technical-devlog-scribe",
-  "description": "Generates a highly structured, objective technical summary of a development session."
+  "name": "technical-devlog-scribe"
 }