docs(release-plan): scrub stale Sigstore §8 dependency from row 3.2

The 2026-05-13 §8 dry-run is closed (RELEASE_CHECKLIST.md §8
header reads "[x] DONE — verified 2026-05-13 on tag
v0.0.0-sigstore-test"), so PUBLIC_RELEASE_PLAN.md row 3.2 should
not list it as an outstanding acceptance criterion. The previous
phrasing implied §8 was still pending and would gate v1.0.0-rc1.

New acceptance criterion for row 3.2:
- Workflow signs the .spl, all CI green
- Note that §8 was completed 2026-05-13 (historical anchor for
  any future reader)
- Run §3.5 Version-Tag Consistency pre-flight before `git tag`
  (the remediation for §8 side-finding #3, also from the
  dry-run)

Estimate revised from "30 min + 1 hr Sigstore E2E" to "30 min"
since the §8 work is no longer in scope for this row.

Memory entries `project_post_rings_reminders.md` + MEMORY.md
index line updated in the same session to reflect the closure,
but those are personal Claude memory (not git-tracked).

Author: RelativisticJet

docs/PUBLIC_RELEASE_PLAN.md

@@ -228,7 +228,7 @@ These nice-to-haves were surfaced during the 2026-05-15 gap review alongside G2-
 | # | Task | Acceptance | Est. |
 |---|------|------------|------|
 | 3.1 | Pre-public review — read every file from an outsider's perspective | Walkthrough checklist documented in `docs/<PRE_PUBLIC_AUDIT>.md`. Anything sensitive or unprofessional flagged | 1 day |
-| 3.2 | Cut v1.0.0-rc1 release tag, private | Workflow signs the .spl, all CI green, Sigstore E2E verification per §8 of `docs/RELEASE_CHECKLIST.md` completed (legitimate + tampered artifact verification, Rekor entry confirmed) | 30 min + 1 hr Sigstore E2E |
+| 3.2 | Cut v1.0.0-rc1 release tag, private | Workflow signs the .spl, all CI green. (Sigstore E2E one-shot — §8 of `docs/RELEASE_CHECKLIST.md` — was completed on throwaway tag `v0.0.0-sigstore-test` on 2026-05-13; this row no longer carries the §8 dependency. Also runs the §3.5 Version-Tag Consistency pre-flight before `git tag`.) | 30 min |
 | 3.3 | Internal verification of rc1 .spl: install on clean Splunk container, run full E2E suite, manually verify major UI flows | Full install + uninstall + reinstall works clean | 4 hr |
 | 3.4 | **Flip repo private → public** (one-way without history rewrite) | GitHub repo visibility = public | 5 min |
 | 3.5 | Community pre-announcement on Splunk dev Slack, Splunk Lantern, dev.splunk.com forums | Posts published, links shared | 1-2 hr |