chore: close out 4.8.7 release readiness

[Activer007]

Summary

• bump VERSION to 4.8.7-SUPAI
• align release-readiness feature rows with current defaults: context_engine enforce and tool_calling enabled
• document tool_calling in release readiness criteria and add test coverage for readiness defaults

Release check

• Decision from generated readiness report: GO-WITH-FLAGS-OFF
• Live test: skipped because no usable live account credentials are available; offline current-input smoke passed

Validation

• ./scripts/lint.sh
• ./tests/scripts/check-refactor-line-gate.sh
• ./tests/scripts/run-unit-all.sh
• npm run build --prefix webui
• ./tests/scripts/run-offline-current-input-smoke.sh
• go run ./cmd/release-readiness --version 4.8.7-SUPAI --branch current --scope "4.8.7-SUPAI release closeout" --out artifacts/release-readiness/4.8.7-SUPAI.md --json artifacts/release-readiness/4.8.7-SUPAI.json --lint-result pass --refactor-result pass --unit-result pass --webui-build-result pass --offline-current-input-smoke-result pass --live-result skip --live-skip-reason "no usable live account credentials available; offline current-input smoke passed"