docs: move website ADR + plan into a dedicated docs/website/ namespace

The website subproject drew ADR numbers from the same global integer pool
as main, so every merge risked a collision — this already happened twice
(drafted 0042, bumped to 0044, both landing on numbers main had taken; main
has now used 0044 for relationship visualization). Give the website its own
ADR namespace so the two never compete again.

- docs/adr/0044-public-website-...md
    -> docs/website/adr/20260604-adr-website-001.md  (id: ADR-website-001)
- docs/plans/20260604-adr-0044-website.md
    -> docs/website/plans/20260604-website-implementation-plan.md
- new docs/website/adr/README.md index (dated <date>-adr-website-<NNN> seq)
- docs/adr/README.md: drop the 0044 entry, add a namespace pointer in the
  intro (keeps the list tail == merge-base, so main's 0044 merges cleanly)
- ADR-0000: record the subproject-ADR-namespace convention
- update references in STYLE.md, astro.config.mjs, the plan body

Handoff files left untouched as point-in-time history.

website/STYLE.md

@@ -4,7 +4,7 @@ This is the **living** home for documentation authoring conventions for the
 RDBMS Playground website. It grows as we write.
 
 - **Binding rules** come from
-  [ADR-0044 §7](../docs/adr/0044-public-website-and-documentation-site.md);
+  [ADR-website-001 §7](../docs/website/adr/20260604-adr-website-001.md);
   this guide must not contradict them. If a convention is significant,
   durable, or contested, it is decided in an **ADR** (new or amended), and
   this guide references it. Finer, settled conventions live here directly.
@@ -17,7 +17,7 @@ Status tags used below: **[DECIDED]** (binding or settled) ·
 
 ---
 
-## Terminology & wording  [DECIDED — ADR-0044 §7]
+## Terminology & wording  [DECIDED — ADR-website-001 §7]
 
 - **No "DSL".** It is internal jargon. Use **simple mode** (the playground's
   keyword command language) and **advanced mode** (SQL).
@@ -66,7 +66,7 @@ Ground every reference page in source — `parse.usage.*` and `help.*` in
 `src/friendly/strings/en-US.yaml`, `src/dsl/command.rs`, `src/dsl/types.rs`
 — never paraphrase grammar from memory.
 
-## "Planned / not yet available" callouts  [DECIDED — ADR-0044 §7]
+## "Planned / not yet available" callouts  [DECIDED — ADR-website-001 §7]
 
 Any capability that is not yet fully implemented is **omitted** or carries a
 clear callout — never presented as shipped. Standard form: a Starlight aside
@@ -127,7 +127,7 @@ a small standalone example, not by complicating this schema.
 - Pair a hero/landing cast with a text transcript or the equivalent docs
   snippet (accessibility + SEO).
 - Recorded via a scripted-input driver for paced, re-recordable sessions
-  (ADR-0044 §2; recipe in `README.md`). **[OPEN]**: cast file naming,
+  (ADR-website-001 §2; recipe in `README.md`). **[OPEN]**: cast file naming,
   fixed terminal size, light/dark theme handling.
 
 ## Formatting  [DECIDED, refine]