fix(sdk): eliminate OpenAPI cleanup race during generation

[LeXwDeX]

Why

The DAG core gate completed its test phases but failed once during SDK generation because the temporary openapi.json file disappeared before cleanup. An immediate rerun and standalone generated-output check passed with no drift. The one-off failure is not a release regression, but it can make mandatory SDK freshness and DAG gates flaky.

What to build

Make OpenAPI generation and temporary-file ownership race-free. Cleanup should be idempotent only after the lifecycle and ownership are understood; it must not hide an earlier generation failure or permit concurrent runs to consume or delete each other’s temporary input.

Reproduction evidence

• During bun run test:dag-core, SDK generation printed all generated files and then failed while removing openapi.json because the file no longer existed.
• The full gate rerun passed, standalone bun run check:generated passed, and the worktree remained clean.
• The failure has only been observed once so far; treat the missing-file race as the hypothesis to prove, not an established root cause.

Acceptance criteria

• Temporary OpenAPI artifacts have explicit per-run ownership and cannot collide across concurrent or repeated generators.
• Cleanup succeeds when generation succeeds and does not mask generation or schema failures.
• Repeated and concurrent SDK generation completes without missing-file failures.
• bun run check:generated and bun run test:dag-core pass with a clean worktree after completion.
• A regression test or deterministic stress check demonstrates the previously vulnerable lifecycle.

Blocked by

None - can start immediately.

[LeXwDeX]

SpecGit delivery branch: feat/475-sdk-openapi-race (draft pull request #483).

[LeXwDeX]

Delivered by PR #485 (merged into dev 2026-09-01, Closes #475) after draft PR #483 was superseded. Auto-close did not fire because the merge target was dev, not the default branch. Promoted to main via PR #518.