Refactor startup planning for direct tests

[benvinegar]

Summary

• extract entrypoint branching into a dedicated startup planner
• keep main.tsx focused on side effects like pager output, renderer setup, and MCP daemon startup
• add direct tests for help, MCP daemon, plain-text pager, diff-like pager normalization, and controlling-terminal attachment

Testing

• bun install --frozen-lockfile
• bun test test/startup.test.ts test/help-output.test.ts
• bun run typecheck
• bun test --coverage