refactor(acp)!: give AcpAgent its own launch config

[benbrandt]

Replace the reused MCP wire-schema type with an SDK-owned stdio process configuration. Update JSON parsing, Yopo, tests, changelog, and the 2.0 migration guide, and remove obsolete agent constructors.

BREAKING CHANGE: AcpAgent::new now accepts AcpAgentConfig; server and into_server are replaced by config and into_config; JSON environment variables are a string map; zed_claude_code, zed_codex, and google_gemini are removed.

[Copilot]

Pull request overview

This PR refactors AcpAgent to use an SDK-owned process launch configuration (AcpAgentConfig) instead of reusing the MCP wire-schema types, updating parsing, example client behavior, tests, and 2.0 migration documentation to match the new API and JSON shape.

Changes:

• Introduces AcpAgentConfig and updates AcpAgent APIs to use config()/into_config() and the new JSON format (env as a string map).
• Updates YOPO argument handling to preserve shell argument boundaries while still supporting single-argument JSON configuration (and adds targeted tests).
• Updates changelog and the 2.0 migration guide to document the breaking changes and removals of obsolete constructors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/yopo/src/main.rs	Updates CLI/docs and parsing logic for agent args (JSON vs argv), adds tests for new behavior.
src/agent-client-protocol/src/lib.rs	Re-exports AcpAgentConfig from the crate root.
src/agent-client-protocol/src/acp_agent.rs	Implements AcpAgentConfig, migrates AcpAgent internals/parsing/tests from schema types to SDK-local config.
src/agent-client-protocol/CHANGELOG.md	Documents the breaking API and configuration shape changes.
md/migration_v2.0.md	Extends the 2.0 migration guide with the new AcpAgentConfig API and JSON/env changes.