Add configurable default save name command

[liurenfeng94-ops]

Addresses #20.

This adds a new tsperf.tracer.saveNameCommand setting so users can generate the default save name from a shell command. The command runs in the workspace root, uses the first line of stdout, and falls back to default on empty output or command failure.

Changes:

• add saveNameCommand to the config schema and generated contributes
• normalize command output into a safe relative save-name path
• compute the default save name asynchronously during app initialization
• add unit tests for save-name normalization

Verified with:

• pnpm vitest run test/index.test.ts test/save-name.test.ts
• pnpm typecheck
• pnpm exec eslint src/appState.ts src/configuration.ts src/constants.ts src/saveName.ts src/storage.ts scripts/generate-contributes.ts test/save-name.test.ts
• pnpm build