docs(acp): IDE integration guide and validation for subagent visibility

[bug-ops]

Part of epic: #1007

Depends on: #1008, #1009, #1010

Problem

There is no documentation on how to connect an IDE (Zed, VS Code with ACP extension) to Zeph's ACP server, and no validation that the subagent visibility features work end-to-end.

Tasks

• Connect Zed IDE to Zeph ACP stdio transport and verify subagent nesting in chat view
• Verify terminal output streaming appears in tool cards in real time
• Verify file navigation ("agent following") when read_file/write_file fires
• Add docs/ide-integration.md covering:
  • Prerequisites (Zed version, ACP extension)
  • Config snippet for enabling ACP in config.toml
  • How to launch Zeph as ACP stdio server
  • Subagent visibility feature explanation
• Update root README.md IDE section via /readme-generator

Files

• docs/ide-integration.md — new file
• README.md — update IDE section