Prepare stable release v0.1.5

[dev-pi2pie]

Summary

This PR prepares the stable 0.1.5 release from dev into main.

What Changed

• bumped the package version and embedded version to stable 0.1.5
• replaced softprops/action-gh-release with a GitHub CLI-based release step
• made the release job rerun-safe by editing an existing release when the tag already exists
• split the config CLI tests into smaller suites and cleaned up related test hygiene
• added targeted coverage for:
  • config discovery
  • env-driven config
  • config loading and parsing
  • user config directory behavior
  • inspect/config precedence
  • warning behavior
  • path override behavior
• aligned package metadata so the published TypeScript peer range supports both TS 5 and TS 6
• refreshed docs and schema notes to match current stable behavior

Why

• 0.1.5 is the next stable cut from the current dev branch work
• the release workflow no longer depends on a JavaScript action that still warns under the Node 20 to Node 24 Actions transition
• the test split improves maintainability and makes config behavior easier to verify in isolation
• widening the TypeScript peer range avoids incorrect peer dependency warnings for TS 6 consumers

Validation

• reviewed the branch diff against main
• verified the stable package version metadata is set to 0.1.5
• verified the release workflow now uses gh release create / gh release edit
• verified peerDependencies.typescript now matches the TS 6 toolchain direction

Notes

• this PR includes test restructuring and documentation updates alongside the stable release prep
• if you want a narrower release PR, the test/documentation cleanup could be split from the versioning and release workflow changes