AI-assisted message generation for commits and change requests

[ed-irl-codebot]

Problem

Commit messages and PR titles/bodies are written by hand. Users who
want to delegate that to a script or an AI assistant have no hook in
git-spice to do so, and no way to have a generated message update an
existing change request as a stack evolves.

Proposal

Add user-configurable message generation: a spice.messageGenerator
config plus a --fill/-c flag that wires a script into
commit create, commit amend, branch create, branch submit, and
branch squash. The script generates commit messages or PR
titles/bodies from the change context.

• New msggen package runs scripts in shebang or sh -c mode and
  forwards the invoking command's argv as positional parameters.
  Scripts run in the repo root with context env vars
  (GS_MESSAGE_KIND, GS_MESSAGE_UPDATE, GS_BRANCH, GS_BASE,
  GS_MESSAGE, GS_TITLE, GS_BODY).
• --fill composes with -m/-F, so a generator failure falls back to
  a user-supplied message.
• Forge support for editing a change request's subject and body
  (GitHub, GitLab, Bitbucket, ShamHub) so a generator can iterate on an
  open CR, not just new ones.

The shell-level contract is shared with the integration and restack
conflict resolvers (see #3 / scriptrun) so all script-driven recipes
behave consistently.

Tracked by

• feat(worktree): add worktree management commands and per-worktree trunks #1170 — AI message generation feature

[abhinav]

For visibility, there are currently two implementations of this under discussion:

• submit: add --fill-from parameter for stack submission commands #1158 by @water-sucks — isolated to only submit commands, with a --fill-from parameter
• feat(worktree): add worktree management commands and per-worktree trunks #1170 by @ed-irl (PR description seems to have messed up) — same functionality used for all local commits and submission

While I have yet to evaluate each of these in detail, one thing worth noting that the outcomes of the two PRs are slightly different:

• --fill-from is meant to be agent-invoked. commit operations can currently all be invoked by an LLM agent directly, passed commit information non-interactively. gs branch submit can be invoked by an LLM agent and passed a title and body non-interactively. It is currently not possible for an agent to invoke multi-branch submit, e.g. gs stack submit for unsubmitted branches, as title/body query for that route requires an interactive terminal.
• spice.messageGenerator, on the other hand, looks like it's meant to be agent-and-user invoked (or just user-invoked?). I run gs commit create --fill, and it'll shell out to $harness -p '...' to get me back a commit message.