feat: run vibe and Acme Execute in the background

[nooga]

Summary

• I/O for vibe and Acme Execute runs in a let-go future; the editor keeps reading keys while jobs are in flight (term/key-pending? + a short sleep, blocking read-chord when idle).
• Results apply on the main thread via drain-jobs ({:ok :err} so a thrown future cannot vanish as nil). Goroutines never touch buffers.
• C-g discards in-flight results (does not kill HTTP/os/sh); the mode line shows vibing... / N jobs after typing has cleared the echo area. :pending-task stays for main-thread paint-then-run work.

Test plan

• lg test/run.lg (340 tests, 1493 pass)
• C-c C-v on a (vibe "...") form: echo says vibing, you can still type, splice lands when the model returns
• F7 / Acme Execute on a slow rc command: same — editor stays interactive, output appends to +Errors
• C-g while a job is running: result is dropped, no splice / no +Errors append
• Two overlapping vibes: each uses its own *vibe-context*, no crossed prompts

Made with Cursor