feat(cli): support inline mcp add args

[opencode-agent]

Summary

• add inline argument support for opencode mcp add [name] [args...]
• support the core inline options: --type, --env, --header, and --global
• parse remote servers from a single URL positional and local servers from -- <command> [args...]
• document Amp-style local command usage and add CLI coverage for local/remote inline adds plus the required command separator

Fixes #29827

Tests

• bun typecheck
• bun test test/cli/mcp.test.ts test/cli/help/help-snapshots.test.ts --timeout 180000
• pre-push bun turbo typecheck