Add pj deepseek command to query account balance and
pj deepseek setkey to store API key in local cache.
Also extract Python launcher into its own module (pj_python.nu)
and add common aliases (g, c, tree, myip, ports, df, reload, edit).
- switch completion module imports to export use so externs propagate
- extend managed completion command set and required file checks
- rename prompt static cache env var to PJ_LEFT_PROMPT_STATIC_PART
- load completion module via use in nu_polyjuice