Context Store First-Run And Cleanup UX Tasks

• Decide exact no-argument context-store setup behavior for TTY, non-TTY, and --json invocations.
• Design the interactive setup prompts for store id, target path, and Git initialization.
• Define target-path safety behavior for managed defaults, explicit paths, paths inside existing Git repos, and non-empty directories.
• Implement the interactive setup flow without changing deterministic non-interactive behavior.
• Decide whether the cleanup surface is unregister, remove, or both.
• Define cleanup semantics for "forget local registration" versus "delete local files too".
• Implement local registry cleanup with explicit confirmation before file deletion.
• Add human output that stays small and JSON output that reports exact setup and cleanup state without next_commands.
• Keep Git initialization scoped to local git init with no auto-staging, committing, pushing, remote creation, or team policy.
• Add focused tests for setup prompts, non-interactive failures, path safety, registry cleanup, and JSON output.
• Update beta docs and agent playbook references for first-run setup and cleanup.