docs(spec): write ADR documenting the fork-and-sync OpenAPI spec ownership model

[dougborg]

Context

PR #47 implemented the fork-and-sync model for `docs/statuspro-openapi.yaml`: the local file is the source of truth, `scripts/sync_openapi_spec.py` (and the weekly CI workflow in #48) pulls upstream and writes a normalized snapshot to `docs/statuspro-openapi.upstream.yaml` for diff-based reconciliation.

The original acceptance criterion on #29 included writing an ADR documenting the choice. That deliverable wasn't completed when the issue was closed.

Proposed change

Add `docs/adr/NNNN-spec-fork-and-sync-ownership.md` (use the next available ADR number) following the project's existing ADR template. Should cover:

• Context: spec was originally synced from upstream, no clean way to evolve locally without drift.
• Decision: fork-and-sync. Local spec is source of truth; weekly CI pulls upstream into a separate snapshot.
• Consequences:
  • Pro: free to add params/endpoints the server already supports but upstream hasn't documented (`page` from fix(client): repair list_orders pagination and add missing page param #27).
  • Pro: drift surfaces as a PR for human review, not silent breakage.
  • Con: divergence requires manual reconciliation when upstream renames or removes fields.
• Alternatives considered: overlay (separate `upstream` + `overlay` files merged at regen), subordinate (file changes upstream first). Both rejected.

Acceptance

• ADR written under `docs/adr/` with the next sequential number
• Linked from CLAUDE.md and AGENT_WORKFLOW.md where the spec workflow is described
• References the implementation (PR feat(spec): OpenAPI spec sync script + upstream snapshot #47) and the weekly sync workflow (PR chore(ci): add weekly upstream OpenAPI spec sync workflow #48)