Add onboard, forge, and notify plugins

[ApurvBazari]

What

Adds three external-link plugins under Developer Productivity:

• onboard — Analyze existing codebases and generate tailored Claude tooling (CLAUDE.md, path-scoped rules, skills, agents, hooks, CI/CD). Includes /onboard:evolve drift detection and plugin-aware generation.
• forge — Greenfield project scaffolder. 4-phase wizard, stack-agnostic via web search, walking-skeleton mode, session resume. Delegates tooling generation to onboard.
• notify — Cross-platform system notifications for Claude Code hooks. macOS (terminal-notifier) and Linux (notify-send) with duration filtering.

Why

• onboard produces actual Claude artifacts from codebase analysis (most existing tools only recommend).
• forge is an end-to-end greenfield scaffolder that fills a gap in the ecosystem.
• notify is a simple, reliable cross-platform notification helper for hooks.

All three are v1.0+, MIT licensed, documented, and live at https://github.com/ApurvBazari/claude-plugins

Checklist

• Plugins follow standard Claude Code plugin structure
• MIT licensed
• Addresses real use cases
• Doesn't duplicate existing functionality
• Follows the external-link format used by recent submissions (backlog, Maestro, Security Sweep, AgentLint)
• Tested on macOS and Linux
• README and CHANGELOG maintained in source repo