internal/mcp/design: start a full design doc, with stubs

Start writing up a formal design doc for an MCP SDK based on what we've
learned from the prototype. Include discussion of package layout,
jsonrpc2, and transport. Lay out the rest of the design sections,
leaving stub comments describing what they should cover.

Change-Id: Id0e5b408eac8c0acb5ef90a32d0d69473d8e83f0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670536
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

mcp/transport.go

@@ -35,8 +35,7 @@ type Transport interface {
 	Connect(ctx context.Context) (Stream, error)
 }
 
-// A Stream is an abstract bidirectional jsonrpc2 Stream.
-// It is used by [connect] to establish a [jsonrpc2.Connection].
+// A Stream is a bidirectional jsonrpc2 Stream.
 type Stream interface {
 	jsonrpc2.Reader
 	jsonrpc2.Writer