feat(sui-tools): switch edit tool to unified diffs

[pranc1ngpegasus]

What changed

• Replaced SEARCH/REPLACE editing with single-file Git unified diff input.
• Validate unified diff syntax and hunk line counts with gitpatch.
• Persist a 0600 write-ahead patch, run git apply --check, then git apply.
• Support /dev/null create/delete diffs and reject renames, multi-file patches, and worktree-external paths.
• Centralize the gitpatch dependency in workspace dependencies.

Why

The edit tool now uses the same patch format as Git, making hunk boundaries explicit and allowing Git to perform the final safe application.

Validation

• cargo test -p sui-tools (52 tests + doctest)
• cargo clippy -p sui-tools --all-targets -- -D warnings
• cargo check --workspace
• git diff --check