[SanAntonio021]

Behavior

Add optional exact-commit selection to Skill installation and updates:

cc-switch skills install owner/repo:skills/name --source-commit <40-hex-sha>
cc-switch skills update <installed-skill> --source-commit <40-hex-sha>

• --source-commit accepts only a full 40-character hexadecimal SHA and normalizes it to lowercase.
• Commit-pinned installs require an explicit owner/repo:path. If the Skill is already installed, the command directs the user to skills update.
• Commit-pinned updates accept one repository-backed Skill only, conflict with --all, and reject local Skills.
• The selected SHA is recorded in the existing readme_url, so nested source paths remain unambiguous without a database schema change.
• The selection applies only to that command. A later update without --source-commit continues to follow the stored branch or repository default branch. An exact update preserves existing branch tracking.
• Existing install, update, check-update, TUI, and database behavior remains unchanged outside the new opt-in path.

The PR also gates two existing Unix-only OpenClaw symlink tests with #[cfg(unix)]. They currently prevent the Windows cargo test --lib --no-default-features --no-run CI step from compiling. Product code is unchanged by this guard.

Validation

Windows, Rust 1.91.1:

• cargo fmt --check
• git diff --check
• cargo check --lib --no-default-features
• cargo test --lib --no-default-features --no-run
• cargo check --bin cc-switch
• 8 focused tests covering CLI parsing, SHA validation, commit archive URLs, nested-path disambiguation, duplicate installation, repository-only updates, and a local HTTP archive sequence for exact install, exact update, and a subsequent normal branch update.

The complete test suite was not run. Existing repository warnings remain unchanged.

Scope

This PR intentionally does not include local-directory import, cross-process locking, runtime-file exclusion, a transactional file engine, automatic crash recovery, TUI changes, or a database schema bump.