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/date-time/plugin.json

@@ -0,0 +1,6 @@
+{
+  "name": "date-time",
+  "description": "Retrieves the exact current date and time for time-sensitive queries, scheduling, duration calculations, and validating chronological context.",
+  "version": "1.0.0",
+  "author": "Rootiest"
+}

plugins/date-time/skills/date-time/SKILL.md

@@ -0,0 +1,43 @@
+---
+name: date-time
+description: Retrieves the exact current date and time for time-sensitive queries, scheduling, duration calculations, and validating chronological context.
+version: 1.0.0
+user-invocable: true
+author: Rootiest
+---
+
+# Current Date and Time Retrieval
+
+## Purpose
+Enables the assistant to retrieve the exact, real-time current date and time when addressing time-sensitive queries, scheduling tasks, calculating durations, or validating chronological context.
+
+## Trigger Conditions
+Activate this skill whenever the user's prompt:
+* Explicitly asks for the current date, day, time, or year.
+* References relative time expressions (e.g., "today", "yesterday", "next week", "recently").
+* Requires checking if an event has already occurred or is upcoming relative to the present moment.
+* Needs to calculate an age, duration, or countdown from the present day.
+
+## Tool Definition
+
+### `get_current_datetime`
+* **Description**: Executes the system `date` command to fetch the current local timestamp, timezone, and calendar date.
+* **Parameters**: None required.
+
+## Execution Workflow
+1. **Detect**: Recognize a time-sensitive trigger in the user's input.
+2. **Call**: Invoke the `get_current_datetime` tool before generating the final response.
+3. **Process**: Use the returned timestamp to anchor your temporal reasoning.
+4. **Respond**: Deliver an accurate answer reflecting the retrieved date/time naturally, without explicitly explaining that a tool was used unless asked.
+
+## Examples
+
+### Example 1
+* **User**: "What day of the week is it today?"
+* **Assistant Action**: Invoke `get_current_datetime`.
+* **Response**: "Today is [Day of Week], [Date]."
+
+### Example 2
+* **User**: "Is the 2026 World Cup happening this month?"
+* **Assistant Action**: Invoke `get_current_datetime`.
+* **Response**: Evaluates current month/year against the tournament schedule to provide an accurate "yes/no" or countdown.