* fix(usage): qualify estimated quota percentages

Preserve bounded CLI usage confidence through normalization, privacy, retained usage and quota-cache persistence. Qualify panel and popup percentages and exclude estimated or unknown confidence from quota automation and AI quota facts.

Documentation: update usage, AI Insights and development guides plus Unreleased; remove the completed estimated-confidence TODO entry. Keep the separate provider-icons backlog open.

* fix(usage): reject explicit empty confidence

Read confidence at CLI and cache boundaries with field-presence semantics: explicit empty or undefined values are unknown, and only an absent field keeps legacy behavior. Omit the internal legacy marker from saved caches so their round trip preserves field absence.

Documentation: clarify raw confidence and cache semantics in usage and development guides; extend the existing Unreleased entry. Add direct regressions for empty input, explicit undefined, and absent-field cache compatibility.