add instructions how to integrate new agents

[Soph]

Adds instruction how to implement new agents, should be usable for humans and agents.

[cursor]

PR Summary

Low Risk
Documentation-only change with no runtime or behavioral impact on the CLI.

Overview
Adds a new docs/architecture/agent-guide.md that documents how to integrate new agents into the Entire CLI, including the required Agent interface methods, how to map native hooks via ParseHookEvent, and how to register agents.

The guide also covers optional interfaces (hooks installation, transcript analysis, token usage, subagent extraction, file watching), recommended hook-install patterns, transcript chunking/reassembly guidance, common pitfalls, and example code + test templates for a minimal agent skeleton.

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[Copilot]

Pull request overview

Adds a new architecture guide documenting how to implement and integrate new AI agents into Entire CLI’s agent abstraction layer (hooks → normalized lifecycle events → framework orchestration), intended to be usable by both humans and agents.

Changes:

• Introduces a comprehensive “Agent Implementation Guide” covering required agent.Agent methods, optional interfaces, and event mapping.
• Provides implementation templates for an example agent package (types, lifecycle parsing, hook installation patterns, and tests).
• Documents transcript formats (JSONL/JSON), chunking helpers, and common pitfalls (repo root handling, go-git caveats, nil-event behavior).