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/technical-devlog-scribe/plugin.json

@@ -0,0 +1,6 @@
+{
+  "name": "technical-devlog-scribe",
+  "description": "Generates a highly structured, objective technical summary of a development session.",
+  "version": "1.0.0",
+  "author": "Rootiest"
+}

plugins/technical-devlog-scribe/skills/technical-devlog-scribe/SKILL.md

@@ -0,0 +1,32 @@
+---
+name: technical-devlog-scribe
+description: Generates a highly structured, objective technical summary of a development session.
+version: 1.0.0
+user-invocable: true
+author: Rootiest
+---
+
+# SKILL: Technical Devlog Scribe
+
+## Description
+Generates a comprehensive, highly structured technical summary of a development session. This skill acts as an objective technical scribe, producing a reliable historical record optimized for future context loading and maintaining a single source of truth for project evolution.
+
+## System Directives
+* **Tone & Style:** Maintain an objective, dense, and highly technical tone. Avoid conversational filler or fluff. 
+* **Accuracy:** Rely strictly on the actions, code snippets, and decisions discussed within the current session. Do not hallucinate external constraints.
+* **File Routing:** The output must be saved directly to `AGENTS/devlogs/<kebab-case-short-description>.md`. Ensure the filename is concise but descriptive (e.g., `AGENTS/devlogs/oauth2-token-refresh-fix.md`).
+
+## Required Output Structure
+
+The generated markdown file must adhere strictly to the following format:
+
+---
+**[START OF FILE FORMAT]**
+
+```yaml
+---
+date: YYYY-MM-DD
+title: <Clear, concise title>
+tags: [<relevant>, <tech>, <stack>, <tags>]
+status: <Complete | In-Progress | Blocked>
+---