integration: Move regeneration out of merge driver

[ed-irl]

integration: Move regeneration out of merge driver

The regenerate merge driver previously ran generators (mise generate,
go test -update) from inside git's merge process. Those generators
side-effect on many files in one pass, and git aborts subsequent merge
steps with 'your local changes would be overwritten' when it sees a
worktree dirtied by an earlier driver invocation. The existing
stage_outputs workaround called git add from inside the driver to
stage the side effects, but git holds the index lock for the duration
of the merge — git add silently failed with rc=128 every time.

This commit moves regeneration out of the merge driver entirely:

• The driver becomes take-incoming + log: cp the incoming version
  into place, append the path to the file named by
  $GS_INTEGRATION_REGEN_LOG. No side effects.

• gs runs all tip merges with that env var pointing at a per-merge
  temp file, accumulates the deduplicated path list across the
  rebuild, and invokes .gs/integration-regenerate (a project-level
  executable at the repo root) with the list on stdin. The script
  chooses what to regenerate based on which categories of files
  appear in its input — so a rebuild with no derived conflicts
  pays one process spawn and exits.

• Whatever the script wrote to the worktree gets folded into the
  last merge commit via git add -A && git commit --amend --no-edit
  --allow-empty.

The contract is well-orthogonalized: gs hands the script an
allow-listed list of paths (only files tagged merge=regenerate in
.gitattributes can appear there), the script returns by writing to
the worktree, gs takes care of the commit-amend wrap-up. Project
generator knowledge stays in the project's script; gs doesn't carry
a dispatch table.

integration regenerate: Regenerate CLI reference

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat: Add worktree commands with per-worktree trunks #1249
    • feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1188
      • worktree: Add worktree commands, per-worktree trunks, and worktree-aware filtering #1165
        • feat: Add worktree commands, per-worktree trunks, and worktree filters #1172
          • worktree: Add worktree management commands, per-worktree trunks, and worktree-aware filtering #1173
            • integration: Wire auto-rebuild hook on restack commands #1174
              • integration: Auto-rebuild integration branch after restack #1175
                • integration: Auto-rebuild integration branch on restack #1176
                  • integration: Add AI-driven conflict resolution #1251
                  • integration: Move regeneration out of merge driver #1252 ◀
                  • integration: accept-incoming fallback after auto-resolve #1253
                  • integration: submit tip downstacks alongside the integration branch #1254
                  • integration: surface integration branch and tips in gs ll #1255
                  • integration: short-circuit gs branch checkout for the integration #1256
                  • integration: gs ss from integration submits all tip stacks #1257
                  • integration: tip clean prunes subsumed tips #1258
                  • integration: tip advance moves tips to upstack leaves #1259
                  • integration: rebuild fixes and reliability improvements #1260

_{Change managed by git-spice.}