docs: archive the finished backlogs (RD-30)

Two backlog trees are complete: `docs/project/backlog/` (75 files, every
WP done) and `docs/project/refactor-backlog-setup/` (the arc before it).
Move both under `docs/project/archive/` with `git mv`, so history stays
intact through `git log --follow`. `SHOWCASE-ROADMAP.md` moves with them,
because it points at the now-archived backlog README.

Add `docs/project/archive/README.md`. It states that these trees are
historical and names the two directories that are still live.

Repoint every inbound reference named in RD-30's Files table: CLAUDE.md,
the root README, both backend READMEs, `LetterHtml.cs`, `a11y.mdx`, the
`document-feature` and `new-ssp` skills, and the readable-codebase PLAN,
README, and RD-19 ticket. Fix two upward-relative links inside the moved
WP files (WP-68, WP-69) that gained a directory level and would otherwise
break. Repoint `.prettierignore`'s two agent-prompt exclusions to their
new path, so prettier keeps leaving those files' exact wording alone.

Mark RD-30 done and check off its acceptance criteria; flip its README
row to done.

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

docs/project/archive/refactor-backlog-setup/agents/06-adr-conformance.prompt.md

@@ -0,0 +1,16 @@
+MODEL: Opus
+OUTPUT FILE: /refactor-backlog/06-adr-conformance.md
+DEPENDS ON: 00-baseline.md (complete)
+
+---
+
+[Insert contents of _persistence-protocol.md here]
+
+AGENT: ADR-Conformance Agent
+
+Read all ADRs/docs in the repo. Compare code against each. Two outcomes per
+deviation:
+(a) code violates a correct ADR → refactoring ticket, cite ADR.
+(b) ADR itself appears outdated/wrong given current code or constraints → propose
+    an ADR amendment as a separate ticket type ("ADR-fix"), with rationale — not
+    a code ticket.