ci(oasdiff): delete broken workflow, file follow-up (#170)

The oasdiff workflow references `go run ./cmd/openapi` which does not
exist (there is no cmd/openapi/ — only cmd/smoke-buildinfo/). The fall-
back chain (`go build ./... && ./api --emit-openapi`) also doesn't
work — the api binary doesn't accept --emit-openapi. In practice the
workflow's 'Build openapi.json from PR' step always fell through to the
'WARNING: no openapi-emit binary found' echo, comparing prod's spec to
prod's spec, so it only ever emitted no-op output.

Bug caught by reviews during PR #167 and #168 (mentioned in QA-session
PR-3 brief). `fail-on-diff: false` means it was never an actual gate,
just noise. Delete and file a tracking issue for the proper wire-up.

A real replacement needs ONE of:
  (a) wire api binary to emit openapi via a CLI flag (--emit-openapi),
      then update the workflow to invoke it; or
  (b) add cmd/openapi/main.go that imports internal/handlers/openapi.go
      and writes the spec to stdout, matching the existing `go run
      ./cmd/openapi` invocation; or
  (c) use the live /openapi.json from a build-of-the-PR (run api in
      docker, hit the endpoint) — slowest but no Go-side code change.

Tracking issue filed; until then we have no automated OpenAPI breaking-
change check on PRs. The contract-PR surface checklist (CLAUDE.md
rule 22) remains the manual gate.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805