Add platform-aware cache paths and home/path normalization

[codeforester]

Parent: #35

Goal

Make generic runtime path selection and path redaction correct on POSIX systems, macOS, WSL2, and native Windows.

Scope

• Keep BASE_CLI_CACHE_DIR as the explicit override.
• Use platform-appropriate default cache locations, including Windows user-local data directories.
• Preserve the existing Linux/macOS behavior.
• Replace separator-specific home/path compaction with pathlib-based comparisons.
• Add tests for POSIX, macOS, WSL-style Linux, and Windows path forms.
• Keep platform decisions in a small reusable helper rather than scattering OS checks.

Acceptance criteria

• Windows does not default to ~/.cache.
• Paths are redacted/compacted correctly with both slash styles and drive letters.
• Explicit cache roots remain authoritative on every platform.
• Existing public profile and runtime APIs remain compatible.