Clarify command overrides empty values

[johnpfeiffer]

Summary

Clarifies the commandOverrides contract for interview findings: empty or whitespace-only override values normalize to “no command” and remove the detected command.
This matches the existing synthesizer behavior and the interviewer help guidance for removing placeholder commands.

Also updates the README setup flow to cover steps that tripped me up as a first-time contributor: bun generate:assets before typecheck, ca --version as a smoke test before bun link.

Changes

• Update synthesizeProjectEntry tests to expect empty and whitespace-only command overrides to remove the detected command.
• Add explicit test case for exact empty string ('') alongside existing whitespace-only (' ') case
• Update README setup and verification commands with bun generate:assets and ca --version

Verification

• bun test src/__tests__/interview.spec.ts
• bun run typecheck