Add the worker-host supervisor ownership pipe

[arasovic]

Summary

• add an EOF-only ownership pipe for hidden worker-host role processes on fd 5
• keep supervisor role processes on the existing fd 3/fd 4 boundary
• close inherited child-side handles in the parent and fail closed on partial setup
• make ownership shutdown idempotent and include it in ordered process teardown
• add Unix lifecycle, CLOEXEC, descriptor-order, startup-failure, and cleanup regressions while preserving non-Unix compilation

Verification

• go test -race ./internal/background -run TestRoleProcess -count=20
• go test -race ./internal/background -count=2
• npm run verify
• go build ./...
• GOOS=windows GOARCH=amd64 go test -c ./cmd/pi-worker
• verified no role process remained after the suite
• independent standards review: no actionable findings
• independent spec review: no spec gaps

Mutation proof

The focused tests failed as expected when deliberately changing each invariant in disposable worktrees:

• omit worker fd 5
• swap fd 4 and fd 5
• omit fd 5 close-on-exec
• retain the parent's child-side ownership reader after Start
• mark ownership closed without closing the writer

The unrelated concurrent skill-rules temporary-file collision observed during verification is tracked separately in #202 and is not changed here.

Closes #201