bunfig: load global ~/.bunfig.toml for runtime commands; fall back past $XDG_CONFIG_HOME - #34987

The shallow merge this enables is incomplete for `jsx.runtime`/`jsx.development`: the jsx block in `bunfig.rs` initializes those to `Automatic`/`true` and writes them into `ctx.args.jsx` unconditionally in the `as_mut()` arm (unlike `factory`/`fragment`/`import_source`, which are gated on `!is_empty()`), so a local `bunfig.toml` without a `jsx` key resets a global `jsx = "solid"` back to Automatic. Not a regression — pre-PR the global `jsx` never applied at all for these commands, so observable 

This spawns with `stderr: "pipe"` but the `Promise.all` only drains `stdout` and `exited`, leaving stderr unread — the pattern REVIEW.md → Subprocess tests calls out ("drain pipes concurrently … an unread pipe fills the ~64KB OS buffer and deadlocks the child"). Output here is tiny so it won't actually deadlock, but it discards stderr on failure and diverges from this file's own `run()` helper (line 33) which drains all three. Add `proc.stderr.text()` to the `Promise.all`, or thread `argv0` thro