Three agent builder files used falsy checks that incorrectly nullified
uiSelectedModel when override objects existed but had no model set:
- sisyphus-agent.ts: `?.model ?` → `?.model !== undefined ?`
- atlas-agent.ts: `?.model ?` → `?.model !== undefined ?`
- general-agents.ts: `!override?.model` → `override?.model === undefined`
This caused user model selection in web mode to revert to defaults.
Fixes#2351
Port devxoul's PR #821 feature to current codebase structure.
Supports absolute, relative, ~/home paths with percent-encoding.
Gracefully handles malformed URIs and missing files with warnings.
Co-authored-by: devxoul <devxoul@gmail.com>
YeonGyu-Kim