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.

CLAUDE.md

@@ -165,7 +165,10 @@ Key invariants in the code:
   ADR. In-flight discussion stays in conversation or issues
   until it settles. The ADR-0000 index-upkeep rule applies:
   every ADR change updates `docs/adr/README.md` in the same
-  edit.
+  edit. ADR **numbers** are assigned at merge-to-`main` (draft
+  under a placeholder `ADR-XXXX` / `draft-<slug>.md` on a
+  branch) to avoid cross-branch collisions — see ADR-0000
+  "Numbering discipline".
 - **Issue tracking.** Bugs and enhancements are filed as Gitea
   issues (see *Issue tracking — Gitea via `tea`* below).
   `docs/requirements.md` and the ADRs remain the source of truth