perf: lazy-load commands and show early spinner for instant feedback

[scottlovegrove]

Summary

• Lazy-load command modules: Instead of importing all 19 command modules at startup, src/index.ts now registers lightweight placeholders and only import()s the module for the invoked command. Cuts td --version from ~400ms to ~75ms.
• Early spinner for instant feedback: A "Loading..." spinner starts immediately before the dynamic import, so there's zero visual silence. It seamlessly hands off to each API call's spinner (text updates in-place without flicker), stays running through all API calls via a release-back mechanism, and auto-clears the moment stdout is written to.
• Add getTasksByFilter to spinner map: Commands using getTasksByFilter (td today, td upcoming, td filter show) previously showed no spinner at all during their API call — now they do.

Test plan

• npm run build compiles cleanly
• npm test — all 680 tests pass
• td today shows spinner immediately, stays running through all API calls, clears cleanly before output
• td today --json shows no spinner
• td --version still fast (~75ms, no spinner)
• td --help lists all commands

🤖 Generated with Claude Code

[doist-bot]

This pull request significantly enhances the CLI's performance and user experience by lazy-loading command modules and introducing an early-loading spinner for instant feedback, notably speeding up td --version and ensuring consistent visual cues during API calls. A minor command parsing edge case was noted, where an option's value might be mistaken for a command when using --progress-jsonl with a space-separated argument.

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