docs(adr): renumber SDD adoption ADR 041 -> 042 (collision with renovate ADR)

Two ADR-041 files landed on main in parallel (sdd-adoption and
renovate-runner-setup). Renames the SDD one to 042 and repoints its references
(SPEC_DRIVEN_DEVELOPMENT, constitution, .specify/adrs/README, sdd-gate.yml).
The renovate ADR keeps 041; its references are left untouched. Riding this PR
per request.

Refs #778
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SPEC_DRIVEN_DEVELOPMENT.md

@@ -3,7 +3,7 @@
 How we turn a feature idea into merged, traceable code in this repo. SDD layers a uniform,
 machine-readable front-end onto the workflow we already run (Gitea issues → branch/PR →
 multi-persona review → red/green TDD). It does not replace any of that — see
-[ADR-041](./docs/adr/041-sdd-adoption.md) for the why.
+[ADR-042](./docs/adr/042-sdd-adoption.md) for the why.
 
 - **The rules** live in [`.specify/constitution.md`](./.specify/constitution.md) (humans) and
   [`.specify/AGENTS.md`](./.specify/AGENTS.md) (AI agents, every invocation).
@@ -179,7 +179,7 @@ issue body for you via the Gitea API.)
   when a project-wide rule genuinely changes. Bump the semantic version (MAJOR = rule
   removed/weakened, MINOR = rule added/tightened, PATCH = wording), run the §6 Sync Impact
   review, and let the `constitution-diff` CI job list the files to reconcile. Record the bump
-  in ADR-041's revision log (or a superseding ADR for MAJOR).
+  in ADR-042's revision log (or a superseding ADR for MAJOR).
 - **AGENTS.md** — keep it under 200 lines. It cross-references the constitution; it must never
   duplicate or contradict it.
 - **ADRs** — project-wide/irreversible decisions go in [`docs/adr/`](./docs/adr/) (next free