Document cross-platform support tiers and release readiness

[codeforester]

Parent: #35

Goal

Publish a clear support contract so users know what base-cli supports and what remains consumer-specific.

Scope

• Add a support matrix covering Linux distributions, WSL2, macOS, and native Windows.
• Explain that WSL2 support means Linux-side execution and does not promise Windows path translation.
• Document cache locations, privacy semantics, history locking, shell/terminal expectations, and known limitations.
• State explicitly that native Windows support for base-cli does not imply native Windows support for Base/basectl.
• Update package metadata/release notes when the implementation is complete.

Dependencies

• Follows the implementation and validation work from Add platform-aware cache paths and home/path normalization #36, Make private runtime files and history locking portable across POSIX and Windows #37, Add native Windows compatibility for base-cli testing and terminal behavior #38, and Add Linux distribution and WSL2 validation for base-cli #39.

Acceptance criteria

• README and dedicated docs agree on the same support tiers.
• Users can identify the supported Python/platform combinations and the appropriate bug-report information.
• Release notes call out any compatibility or security-semantic changes.