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/ship-it/plugin.json

@@ -0,0 +1,6 @@
+{
+  "name": "ship-it",
+  "description": "Runs a comprehensive pre-flight audit, syncs the README, and publishes the changes to a new PR.",
+  "version": "1.0.0",
+  "author": "Rootiest"
+}

plugins/ship-it/skills/ship-it/SKILL.md

@@ -0,0 +1,25 @@
+---
+name: ship-it
+description: Runs a comprehensive pre-flight audit, syncs the README, and publishes the changes to a new PR.
+version: 1.0.0
+user-invocable: true
+author: Rootiest
+---
+
+# /ship-it
+
+Instructions:
+Execute the following two phases sequentially. Do not proceed to Phase 2 unless Phase 1 completes successfully.
+
+1. Phase 1: Documentation Sync & Code Audit
+   - Act as the `/docs-sync-audit` skill.
+   - Scan all file changes since the last README edit and update the README to ensure it accurately reflects the current state of the codebase.
+   - Audit all code files for any syntax errors, regressions, or issues.
+   - CRITICAL: If any code errors or breaking issues are discovered during the audit, HALT the workflow immediately and report them to the user. Do not proceed to publishing.
+
+2. Phase 2: Git Publish Workflow
+   - Act as the `/git-publish-workflow` skill.
+   - Create a new, descriptively named git branch.
+   - Stage and commit all pending changes (including the newly updated README from Phase 1).
+   - Push the branch to the remote repository.
+   - Generate a Pull Request (PR) from the new branch into 'main'.