feat(graph): setting to hide stashes from the graph

[patrick-cst1]

Summary

• Adds gitGraphPlus.showStashes (boolean, default true = current behaviour). When false, stashes are not inserted into the graph and commit search results, and no stash walk start points are added. The Stashes sidebar is untouched.

Related Issue

Closes #100

Changes

• LogOptions.includeStashes + log() skip; searchCommits/searchByFile add --exclude=refs/stash when disabled.
• All MainPanel log()/search call sites threaded + config-change refresh.
• Tests: git-service log/search, config reader, MainPanel pass-through.

Type of Change

• New feature
• Test addition or update

Testing

• New tests fail on unmodified main and pass with the change.- npx tsc --noEmit clean.
• Full suite on Windows: 260 pre-existing failures (POSIX-only integration helper + env), 0 new failures vs main; focused tests pass.
• The repo's integration suite only runs on Linux (its helper uses POSIX quoting), CI covers those.