Member-mode close still lands its entry, durably, in the hub queue

[sameera]

As an engineer closing an epic in a member repo of a multi-repo workspace, I want the move to
ephemeral .nexus/tmp/ artifacts to leave the hub migration intact, so that a member close still
ends with exactly one durable, drainable entry in the hub queue rather than a gitignored copy that
never migrates.

Acceptance Criteria

• Given a member-mode close (always non---pr; /nxs.close Phase 7.5) of an issue-sourced
  epic whose close-record.md and analyze-receipt.md were written under .nexus/tmp/, when
  the migration helper runs, then the entry still lands committed in the hub queue with those
  files byte-for-byte — being under a gitignored path must not make the migration copy nothing,
  skip the entry, or fail its verify step.
• Given that migration has succeeded, when close finishes, then the entry exists in
  exactly one place (the hub queue) — no tmp copy is left behind that a later /nxs.distill in
  the member checkout could discover as a second, separately drainable entry (STORY-170.03).
• Given the migrated entry, when /nxs.distill runs in the hub, then the drain-SLO
  report still attributes it to its originating repo from the first range: entry's repo in
  close-record.md, and the introducing-commit age still measures from the migration commit —
  unchanged from today.
• Given single-repo or hub mode, when a local close runs, then no migration happens
  and STORY-170.02's tmp-only behaviour applies unchanged — this story adds no committed artifact
  to those two modes.

Notes

Part of epic #170. Closes the gap the epic's first draft asserted away ("close never runs git commit
itself in local mode") — member mode is non---pr and commits, so "ephemeral" describes where a
member close writes, never where it ends.

[sameera]

Closed in f5f5373