Expand README with task-focused crs-toolchain usage examples

[Copilot]

what

• Added a new Usage Examples section to README.md to provide concrete command recipes beyond installation/self-update.
• Structured examples as a quick command cookbook:
  • Regex workflow (generate, compare, format, update) including single-target, multi-target, stdin, and --all usage
  • Utility commands (fp-finder, renumber-tests) with practical flags (--extended-dictionary, --check, --all)
  • Chore commands (release, update-copyright, create-agenda)
  • CLI ergonomics (--directory, shell completion, GitHub-formatted output/log levels)
• Included copy/paste-ready snippets, for example:

  crs-toolchain regex update 932100 933100-chain1.ra

why

• The issue asked for substantially more usage examples in documentation; README previously lacked practical command coverage.
• Grouped, runnable examples make common CRS maintenance workflows faster to discover and execute from the README alone.

references

• Issue: Add more examples to documentation