Use Git branch names for trace saves

[wndzph9jkb-create]

Summary

• Add an opt-in tsperf.tracer.useGitBranchSaveName setting.
• When enabled, initialize the trace save name from the active VS Code Git repository's current branch or detached HEAD.
• Sanitize Git labels before using them as storage directory names.
• Fall back to the existing default save when Git is unavailable or has no HEAD label.

Why

Closes #20.

Using the current Git branch as the default save name keeps trace files and trace diagnostics aligned with the checked-out code, especially when switching between branches during performance investigations.

Validation

• corepack pnpm typecheck
• corepack pnpm lint
• corepack pnpm exec vitest run
• corepack pnpm run generate:contributes
• cd srcDev && corepack pnpm exec rollup -c
• corepack pnpm exec nuxt build ui
• corepack pnpm exec rollup -c