feat(init): add streamable-http transport option

## Problem `pharos init` only offers `stdio` and `http-sse` as transport options (cmd/init.go:69). The MCP spec has moved to **streamable HTTP** as the preferred remote transport. New servers published today should default to streamable-http, but `pharos init` can't produce one. ## Proposed fix Add `streamable-http` as a third option in the transport selector in `cmd/init.go`. Update `defaultCommand()` to provide a sensible default run command for streamable-http servers. ## Acceptance criteria - [ ] `pharos init` offers `streamable-http` as a transport option - [ ] `defaultCommand()` returns a valid default for streamable-http - [ ] Manifest validation accepts `streamable-http` as a valid transport value - [ ] Existing `stdio` and `http-sse` options unchanged