fix(workflows): address PR #2408 review findings

- Add tests for JSONDecodeError and OSError fallback paths in _resolve_integration_auto()
- Centralize INTEGRATION_JSON constant in workflows/constants.py (zero-dependency module)
- Change changelog reference to (Fixes #2406) for auto-close semantics
- Remove static requires.integrations.any from speckit workflow (incomplete with 'auto')
- Add opencode to integration prompt examples

Author: Markus

CHANGELOG.md

@@ -6,7 +6,7 @@
 
 ### Fixed
 
-- fix(workflows): auto-detect project integration instead of hardcoding copilot default (#2406)
+- fix(workflows): auto-detect project integration instead of hardcoding copilot default (Fixes #2406)
 
 ## [0.8.2] - 2026-04-28