feat(mcp): support project-local configuration

[danielkov]

Summary

Adds automatic project-local .mcp.json discovery and merges it with configured MCP servers. The merged configuration reloads dynamically and is preserved across built-in Kit child processes.

Motivation

Projects can check in their MCP development setup without requiring each user to copy those servers into global Kit configuration or replace their existing configured servers.

Impact

Kit now reads .mcp.json from the effective project root for serve, acp, prompt, and tui. Server definitions use whole-server precedence: plugins, configured MCP file, project .mcp.json, then --mcp-config.

Project-local stdio servers resolve missing or relative cwd values from the .mcp.json directory. Kit also accepts compatible optional type discriminators used by common MCP configuration formats.

Technical details

Layered reloads

Each source is tracked independently. Creating, editing, or deleting the optional project file updates only effectively changed servers, while invalid changes preserve the last valid runtime state.

Child compatibility

Built-in acp.kit children inherit configured and explicit paths separately and rediscover the same project layer. The existing public Runtime::with_mcp_config API retains its legacy single-file behavior.