docs: schedule implementation-plan.md retirement in roadmap

The document is closed: no unique content remains — § D (pre-release
security review) is duplicated in progress.md, security.md and the
CHANGELOG, and B.1-B.3/C.4 were trimmed in 22f86d1. It stays until the
tag only because it describes the release gate.

Record the retirement as a v1.x tail item in roadmap.md with the
concrete steps: archive the file and retarget its references, including
the stale "plan C.4" pointers in Makefile, release.yml and the e2e test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -17,6 +17,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Added
 
+- docs: `docs/roadmap.md` v1.x tail — retire `implementation-plan.md` in the
+  release commit (move to `docs/archive/`, retarget its references in README,
+  docs, Makefile, release workflow and the e2e test comment).
 - docs: `docs/code-review.md` — phase 1.5 plan for optional password encryption
   of full backup (`.spbk`) and domain export (`.spde`); checkbox UI pattern;
   remove session-resurrection-from-backup as accepted risk.