docs: say where a mock for an unexported interface goes

[retr0h]

Implements rescope-go-code-standards task 5.4a. The rule landed in osapi-io/specs#98.

The Test doubles section said generated mocks live in a sibling mocks package. For an unexported interface that is impossible, and the compiler proves it:

imports .../pkg/orchestrator/mocks from orchestrator_test.go
imports .../pkg/orchestrator from renderer.gen.go: import cycle not allowed in test

The mock has to import the package to name the types in the interface; the package's own tests have to import the mock.

What is added

The second case, with its destination scoped to tests:

// generate.go, in the package that declares the interface
package thispackage

//go:generate go tool go.uber.org/mock/mockgen -source=thing.go -destination=thing.gen_test.go -package=thispackage

Scoping is load-bearing rather than tidiness — an unscoped mock puts go.uber.org/mock into the dependency graph of everything that imports the package.

Either way the directives live in a generate.go holding no code, and the generated file carries .gen so a reader knows not to edit it.

The section hashes identically in all five Go repositories.

Docs only. just md-fmt-check passes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #454   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files         484      484           
  Lines       24935    24935           
=======================================
  Hits        24921    24921           
  Misses         14       14           

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 16df030...d971d76. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.