docs: add roadmap for extracting zigts expert into a zigttp agent virtual-module

[srdjan]

Motivation

• Capture a feasibility plan for extracting the existing zigts expert agent loop into a reusable native virtual-module library to enable reuse across CLI, handlers, and future adapters.
• Provide a staged delivery, design constraints, and risk register to guide the extraction while preserving existing semantics and permissions.

Description

• Add a new roadmap document docs/roadmap/aloop-zigttp-virtual-module.md that outlines scope, quick verdict, extraction mapping, candidate module surface, phased delivery plan, and risks.
• Update docs/README.md to include a link to the new roadmap entry under the Roadmap section.
• The roadmap proposes zigttp:agent (with run, stream, complete) and optional zigttp:agent/session surfaces plus interfaces like AgentLoopEngine, ProviderAdapter, ToolRegistry, and AgentSessionStore for phased extraction.

Testing

• No automated tests were run for this documentation-only change.

---

Codex Task