fix: Run Go workspace formatting per module (#14070)
### Description
Fixes TURBO-6072.
Remove the synthesized `go-workspace#format` command, which fails
because `go fmt` rejects cross-module workspace patterns. Root-level
`turbo run format` instead selects the existing per-module tasks:
```text
turbo run format
├── example.com/api#format → go fmt ./... (in apps/api)
└── example.com/lib#format → go fmt ./... (in packages/lib)
```
Keep `go fmt` rather than switching to recursive `gofmt`: package-aware
formatting respects module boundaries and leaves `testdata` and nested
non-member modules untouched. Module formatting remains uncached, and
workspace-level `test` and `lint` commands are unchanged.
Update the Go guide to document per-module formatting and the removal of
the aggregate target.
