docs(compilers/openapi): drop the wrong ir-design citation

[OmarAlJarrah]

Summary

Three doc comments justified keeping openapi.Options whole by citing ir-design §10:

• compilers/openapi/options.go — "Its shape is a published contract (ir-design §10)"
• compilers/openapi/openapi.go (loadOptions) — "whose shape ir-design §10 fixes"
• compilers/openapi/internal/load/load.go (load.Options) — "whose shape is fixed by ir-design §10"

docs/ir-design.md §10 is Servers — the IR's Server struct, its URL template and its
variables. It says nothing about compiler options, and neither does any other section: the
document does not describe an options type under any spelling. Someone following the citation
to check the constraint lands on a struct about server URLs, and cannot tell whether the
constraint lives elsewhere or does not exist.

So the citation is dropped rather than renumbered, because there is nothing to re-point it at.
The claim it was propping up holds on its own terms and now says so directly: openapi.Options
is exported and compilers.Options.FormatOptions is any, so a caller outside the package
constructs that exact type by field name — optionsFrom accepts no other type. options.go
gives the reason once and the other two defer to it rather than restating it, which is what
made a single wrong section number wrong in three places.

Only those three change. The four correct ir-design §10 citations (ir/server.go ×2,
compilers/openapi/meta.go ×2) really are about servers and are left alone, so a blanket
replacement would have been wrong.

Comments only: no behaviour change, no signature change, no golden regeneration.

Note for merge order: #88 edits nolint directives further down internal/load/load.go. The
two changes touch different lines and are independent, but git may still report a textual
conflict in that file depending on which lands first.

Test plan

• Full gate clean: gofmt -l, go vet ./..., golangci-lint run (0 issues), go build ./...,
  ./scripts/check-coverage.sh (100% statements, unchanged).
• grep -rn 'ir-design §10' --include='*.go' . now returns only the four server citations.
• Checked against the doc as it stands rather than the line numbers quoted in the issue, which
  have since moved: ## 10. Servers is at docs/ir-design.md:1480, and
  grep -n 'openapi.Options\|compiler options\|FormatOptions\|Options struct' docs/ir-design.md
  is empty.

No test accompanies this. A section number that is wrong about its subject still resolves to a
real heading, so no mechanical check can tell this apart from a correct citation.

Closes #236