feat(system): add android support and improve temp dir handling

[pgtoopx]

Context

Currently, Gentle AI assumes standard Unix paths (like /tmp) and doesn't recognize android as a supported OS, making it impossible to run on environments like Termux.

Changes

• OS Support: Added android to the supported OS list in internal/system/detect.go.
• Package Manager: Mapped android to use pkg (Termux default) in the platform resolver.
• Portability: Refactored fixed /tmp paths to use os.TempDir() in internal/installcmd/resolver.go.

Impact & Compatibility

• Zero breaking changes: All additions are scoped to the new android detection.
• Improved Robustness: Switching to os.TempDir() benefits all systems by respecting environment-specific temporary directories instead of hardcoding /tmp.

Verification

• Verified on Android via Termux.
• Code remains idiomatic and follows existing patterns.

[Alan-TheGentleman]

Closing this as superseded by #765.\n\nThe Android/Termux direction is valid, but this branch only covers a small slice. #765 is the candidate that carries the fuller platform support path.