integration: Wire auto-rebuild hook on restack commands

[ed-irl]

The four restack commands (branch, stack, upstack, repo) now invoke
IntegrationHandler.MaybeRebuild after a successful restack, so a restack
that moves a tracked tip automatically rebuilds the integration branch
when one is configured. Each command's Run gains an IntegrationHandler
parameter and returns early on restack failure before the hook runs.

The integration_auto_rebuild test script covers the end-to-end flow:
amending a tip drifts its hash, the post-restack hook detects the drift
and rebuilds, and a subsequent restack finds no drift and skips the
rebuild.

merge_wt_test.go now sets git user.name/user.email in its scripts so
the merge tests commit successfully in a config-less environment.

Adds an Added changelog entry.

Part of #1244

[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.}