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.

docs/adr/0000-record-architecture-decisions.md

@@ -58,6 +58,24 @@ compute "next" from a feature branch. A branch that genuinely needs a real
 number up front may instead reserve one by landing a stub index entry on
 `main` first, but placeholder-until-merge is the default.
 
+### Subproject ADR namespaces
+
+A long-lived subproject developed on its own branch can escape the shared
+integer pool entirely by keeping its decision records in a **separate
+namespace**, rather than fighting collisions on every merge. The **website**
+(`docs/website/adr/`) is the first: its ADRs use a dated sequence —
+`<date>-adr-website-<NNN>.md`, referenced in prose as `ADR-website-NNN` —
+and are indexed by their own `docs/website/adr/README.md`. Because the
+date-plus-subproject prefix is disjoint from `main`'s integer sequence, a
+website ADR and a `main` ADR can never claim "the same number" again. (This
+namespace was created on 2026-06-10 after the website's ADR collided with
+`main`'s on consecutive numbers — drafted 0042, bumped to 0044, both times
+landing on a number `main` had taken; the move retired it from the pool as
+**ADR-website-001**.) The main `docs/adr/` index carries a pointer to each
+such namespace. Use this for a new subproject only when it is genuinely
+self-contained and branch-isolated; one-off cross-cutting decisions stay in
+the global sequence.
+
 ## Out-of-scope discipline
 
 ADRs (and the plans they spawn) lean heavily on "out of scope" language.