feat(coding-agent): support shell commands and env vars in auth.json API keys

API keys in auth.json now support the same resolution as models.json:
- Shell command: "\!command" executes and uses stdout (cached)
- Environment variable: uses the value of the named variable
- Literal value: used directly

Extracted shared resolveConfigValue() to new resolve-config-value.ts module.

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- API keys in `auth.json` now support shell command resolution (`!command`) and environment variable lookup, matching the behavior in `models.json`
+
 ## [0.51.6] - 2026-02-04
 
 ### New Features