fix(core): keep local add-ons optional and polish workflows

[jb-thery]

fix

• keep Chat and TTS as optional Core peers so a Core install does not pull local model runtimes
• validate TTS languages consistently and report missing optional packages with actionable install guidance
• stabilize responsive use-case workflows and localize realistic terminal commands in English and French
• align package guides, project documentation, machine-readable discovery, and privacy wording with offline local retrieval boundaries