feat: make primary args positional across all CLIs

[platoneko]

Convert primary arguments from named options (--arg) to positional arguments for a more natural CLI experience.

Changes

Code (26 commands)

• Add positional: true to primary args across 22 TS files and 4 YAML files
• Affected sites: antigravity, bilibili, boss, chaoxing, coupang, grok, hf, instagram, jike, jimeng, linkedin, linux-do, tiktok, twitter, xiaohongshu, youtube

Docs

• Add Arg Design Convention section to CONTRIBUTING.md
• Update download examples in README.md and README.zh-CN.md to use positional syntax
• Add earnings-date to xueqiu command list

Usage examples

opencli twitter followers elonmusk   # was: --user elonmusk
opencli bilibili download BV1xxx     # was: --bvid BV1xxx
opencli grok ask 'hello'             # was: --prompt 'hello'

[jackwener]

Deep review completed. I pushed a follow-up commit to keep filter/config args as named options instead of forcing them positional, and fixed the single-tweet download docs example. Validation passed with npm run typecheck, npm run build, and targeted help checks for the affected commands.