scriptrun: generic script-runner utility for script-driven recipes

[ed-irl-codebot]

Problem

Several planned features need to run user-configured shell scripts and
capture their output — AI message generation, integration conflict
resolution, and restack conflict resolution all want the same
"run a recipe, read what it produced" primitive. Without a shared
utility each would reimplement script execution, output capture, and
exit-code handling slightly differently.

Proposal

Add a small, single-responsibility scriptrun package for executing
user-configured scripts and capturing their output.

• Scripts may be plain shell snippets (run via sh -c) or
  self-contained executables starting with a shebang (written to a temp
  file and executed directly).
• stdout and stderr are captured independently and returned with the
  exit code.
• The package does not interpret output — that is the caller's job —
  keeping it reusable across different output protocols (commit
  messages, conflict resolutions, etc.).
• A non-zero exit is reported in the result rather than as an error, so
  callers can distinguish "script ran and disagreed" from "script could
  not be run at all."

This is foundational infrastructure for the AI message generation and
integration/restack resolver features.

Tracked by

• feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1188 — scriptrun script-runner utility

[abhinav]

scriptrun was merged in #1276. it's being used in the merge feature for spice.merge.command. marking as done.
(PR linked above is not the one that originally implemented scriptrun.)