fix(cache): prevent diff from corrupting disk cache

[rocketman-code]

Summary

• Pass no_cache=true in build_snapshot_from_working_tree so the working-tree side of a diff neither reads nor writes the disk cache, preventing cross-contamination from the git-ref side's reduced graph.

Closes #142

What was happening

When running chainsaw diff <ref> --entry <file>, the git-ref side builds a reduced graph (no node_modules, read from git tree) and writes it to the disk cache. The working-tree side then calls loader::load_graph(entry, false) (no_cache=false), which reads the contaminated cache entry and produces incorrect results. Subsequent chainsaw trace commands also read the corrupted cache until it is manually cleared.

The fix

Change the no_cache argument from false to true in build_snapshot_from_working_tree, matching the behavior already used by build_snapshot_from_ref. This ensures neither side of a diff pollutes the shared cache.

Test plan

• cargo test --workspace passes (all 274 tests)
• cargo clippy --workspace --all-targets -- -D warnings passes (via pre-commit hook)
• Manual: chainsaw diff HEAD~1 --entry <file> followed by chainsaw trace <file> produces correct, consistent results

[rocketman-code]

Closing: patch contents already on main. Branch has zero diff after rebase.