test: cover default plugin options

[ian-pascoe]

Summary

• Add a regression test that initializes the plugin without options and verifies the default tool still executes.
• Verify default system guidance still injects the default prompt and default tool name.
• Keep the generated BRV memory in a separate commit.

Verification

• pnpm test
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm build

[ian-pascoe]

Superseded by #6, rebuilt from latest main.