Add snapshot capture to shadictl around sandboxed command execution in crates/shadictl/src/main.rs.

Detect whether the target working directory is inside a Git repo.

Record pre-run and post-run Git state:
- git rev-parse HEAD
- git status --porcelain
- git diff --binary
- optionally untracked file inventory

Write a session artifact under .tmp or a configurable SHADI runtime directory with:
- command
- cwd
- policy
- timestamps
- before/after Git state
- diff summary

Expose this with a small CLI UX

Keep the first iteration read-only with respect to Git history: do not auto-commit, just capture state and diffs.