Remove the silent two-library default from trigger-add-submodules.sh

[whisper67265]

Problem

scripts/trigger-add-submodules.sh silently defaulted to unordered, json when --submodules was omitted, so operator-triggered dispatches could diverge from raw API behavior. When submodules is omitted from a repository_dispatch payload sent via the GitHub API, the workflow auto-discovers the full library list from boostorg/boost .gitmodules via resolve_add_submodules_names — a behavior the script default masked.

Acceptance Criteria

• trigger-add-submodules.sh exits with an error and usage text when --submodules is missing or empty
• Documentation (GETTING-STARTED.md, endpoint-contract.md) states that --submodules is required on the script while the API field remains optional for auto-discovery
• build_dispatch_json produces correct payloads under jq 1.7 (no collision with the built-in pairs function)
• Bats tests cover trigger-dispatch-common.sh, trigger-add-submodules.sh, and trigger-start-translation.sh with a curl stub for repository_dispatch POSTs

Implementation Notes

• Removed DEFAULT_SUBMODULES and the SUBMODULES="${SUBMODULES:-$DEFAULT_SUBMODULES}" fallback; added an explicit guard before dispatch
• Renamed jq variable from pairs to kv_pairs in build_dispatch_json to avoid jq 1.7 built-in name collision
• Added install_dispatch_curl_stub / extract_dispatch_request_body helpers in tests/helpers/http_mock.bash for offline dispatch testing

References

• CHANGELOG.md
• docs/GETTING-STARTED.md
• docs/endpoint-contract.md
• scripts/trigger-add-submodules.sh
• scripts/trigger-dispatch-common.sh
• tests/helpers/http_mock.bash
• tests/test_getting_started.bats
• tests/test_trigger_add_submodules.bats
• tests/test_trigger_dispatch_common.bats
• tests/test_trigger_start_translation.bats

[whisper67265]

plan file: https://gist.github.com/whisper67265/82951a17c8804c50669e64371ed030e6