Local close writes ephemeral artifacts to .nexus/tmp, never an implied commit

[sameera]

As an engineer closing an epic locally without a PR, I want close-record.md written to
.nexus/tmp/ (alongside the born-here epic.md, when one was materialized there) instead of a path
the spec calls "committed" but nothing actually commits, so that a same-sitting local close needs
no manual git step to hand off to /nxs.distill.

Acceptance Criteria

• Given a local close of an epic resolved from .nexus/tmp (the issue-sourced, Issue-Sourced Planning: Nothing Commits Until Close #114 norm),
  when /nxs.close writes its artifacts, then close-record.md (and epic.md, if this
  is where it was first materialized) is written under .nexus/tmp/, and the Phase 7 checkpoint
  summary describes it as ephemeral hand-off content — never as "committed."
• Given a local close of an old-contract entry with an already-committed epic.md, when
  /nxs.close writes its artifacts, then close-record.md still lands in that committed
  .nexus/queue/ directory, unchanged from today.
• Given --pr mode, when /nxs.close --pr runs, then nothing changes: Phase 0.5 /
  Phase 7.6 keep committing the born-at-close epic.md + close-record.md onto the distill
  branch, because those artifacts have no feature PR of their own to ride.
• Given a local close under the new tmp-only path, when the epic issue is closed, then
  the close comment (Phase 8.2) still carries the close record's Key Decisions + Deviation
  Rationale prose in full — this is now the only durable copy of that rationale for a local
  close, so nothing in the comment-writing step may be skipped or thinned as a result of this
  change.

Notes

Part of epic #170. Depends on STORY-170.01's tmp contract for analyze receipts.

[sameera]

Closed in 3115a96