Add AgentTier to Development section

[hardikvasa]

This PR adds AgentTier to the Tools for Self-Hosting → Development section.

AgentTier is a Kubernetes-native platform that provisions isolated, persistent sandboxes for human developers and AI coding agents from a single Sandbox CRD. Each sandbox is a Pod + PVC + default-deny NetworkPolicy with optional gVisor isolation. It supports both interactive use (browser terminal over WebSocket → SPDY exec) and headless agent workloads (REST POST /configure to install, then SSE-streaming POST /invoke to run). Reference templates ship for Claude Code + AWS Bedrock and a model-free LangGraph agent.

License: Apache-2.0. Self-hosted only — no managed SaaS offering. Distributed as a single Helm chart, with a Python SDK (pip install agenttier), a Go CLI, and a React Web UI.

Why it's useful in this guide: Teams self-hosting their own developer tooling now need a way to also self-host the runtime for AI coding agents (Claude Code, Aider, OpenHands, etc.). AgentTier is one of the few projects that targets the Kubernetes deployment model for this, with built-in governance, audit, and per-session credential injection — fitting the same operational profile readers of this guide are already using for Gitea, GitLab, Sentry, etc.

Compliance with CONTRIBUTING.md:

• ✅ Format [Resource Title](url) — description. with concise but descriptive paragraph, matching the dominant format used throughout the Development section.
• ✅ Personally used / built — disclosure: I'm a contributor to AgentTier, submitting because it fills a self-hosting niche covered nowhere else in the guide.
• ✅ Single suggestion in this PR.
• ✅ Spelling and grammar checked.