Improve prompt cache statusline state

[lroolle]

Summary

• add --cache auto|always|off and richer JSON cache-health state
• keep old numeric state compatibility while sanitizing corrupt state and writing atomically
• show cache rebuild/drop signals without claiming server expire_at; TTL class only appears when usage breakdown proves it
• update README/CHANGELOG and expand cache unit/integration coverage

Verification

• npm exec --yes bats -- t/
• bash -n statusline.sh install.sh t/helpers.bash
• git diff --check

Review notes

• Functionality reviewers checked cache semantics before PR readiness.
• Addressed their findings around malformed state, env-derived TTL uncertainty, misleading cache! docs, current stdin schema limits, and --cache off state writes.