docs(cli): align CLI reference with current implementation

Rewrite the PostgresAI CLI reference to match the actual command tree
in cli/bin/postgres-ai.ts: rename mon quickstart -> mon local-install,
move show-key/remove-key under auth (drop top-level add-key in favour
of `auth --set-key`), use hyphens for issues subcommands, and add the
previously undocumented commands (prepare-db, unprepare-db, checkup,
reports, set-default-project, set-storage-url, mcp install, issues
create/update/update-comment/files/action-items, mon targets test,
mon shell, mon clean, mon check, mon health, mon update, etc.).

Author: NikolayS