docs: ADR numbering discipline — assign numbers at merge-to-main

Codifies the fix for the ADR-0042 cross-branch collision (resolved this
merge by renumbering the website ADR to 0044): ADR numbers are assigned
when a branch merges to main, not at creation. On a branch, draft under
a placeholder (ADR-XXXX title / draft-<slug>.md filename); main's
docs/adr/README.md is the single source of truth for the next free
number.

- ADR-0000: new "Numbering discipline" section.
- CLAUDE.md: pointer to it from the documentation-discipline note.

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

@@ -38,6 +38,26 @@ The index lists ADRs in numerical order. Each entry shows the
 number, title, and — where relevant — status annotations such as
 "Superseded by ADR-NNNN" or "Deprecated".
 
+## Numbering discipline
+
+ADR numbers are a single global sequence, so two branches can each grab
+"the next number" independently and collide on merge. (This happened when
+the `website` branch's ADR-0042 met `main`'s ADR-0042, resolved by
+renumbering the former to ADR-0044.) To prevent it:
+
+**Assign an ADR's number at merge-to-`main`, not at creation.** While the
+work lives on a non-`main` branch, draft the ADR under a placeholder — an
+`ADR-XXXX` title and a `draft-<slug>.md` filename — and reference it that
+way from any plan or notes. Give it the next free number only when the
+branch merges to `main`, renaming the file and updating its references in
+the same step.
+
+A number is "taken" only once it appears in `main`'s `docs/adr/README.md`,
+which is the single source of truth for the next free number — never
+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.
+
 ## Out-of-scope discipline
 
 ADRs (and the plans they spawn) lean heavily on "out of scope" language.