docs(readme): rewrite README for the current CLI

- Drop the removed flashduty-sdk dependency; list go-flashduty, cobra,
  toon-go, yaml.v3 and x/term.
- Replace the hand-maintained command list (which covered about ten of
  the 37 groups and used outdated names) with a group overview by product
  area and the path-to-command rule, and point to --help and the docs
  for the long tail.
- Link the website, CLI docs, API reference, console, blog post and
  related projects; use the flashduty.com and docs.flashduty.com domains.
- Document whoami, update, the environment variables the CLI reads, and
  the Go 1.26 requirement; refresh the version example to v1.5.12.
- Keep README.md and README_zh.md in step.

Author: ysyneu