docs: sync AXguard README with current implementation

[shuvonsec]

Summary

• Synchronize README with current CLI, slash commands, agent install targets, detections, reports, and CI dogfood behavior.
• Document twin, memory, investigate, predict, API, contribute/privacy, and full audit diagnostic artifacts (including soft memory/investigation outputs).
• Correct this repo's Actions example to axguard audit engines + axguard audit cli --fail-on high.

Test plan

• Relative README links resolve in-repo
• Commit messages have no AI or Cursor attribution
• Spot-check Start Using / CLI tables against axguard help and commands/

Do not merge automatically.