integration: submit tip downstacks alongside the integration branch

[ed-irl]

'gs integration submit' previously only force-pushed the integration
branch. Since the integration branch is the union of its configured
tips, publishing it implies publishing those tips. Otherwise users had
to remember to separately submit each tip stack — easy to forget when
gs ints is meant to be the one-shot "publish the integration" command.

For each configured tip, walk its downstack (trunk-exclusive,
dedup'd), then run SubmitBatch in trunk-first topological order so
bases are pushed before the branches that depend on them. After the
tip-stack batch completes, push the integration branch itself with
its existing force-with-lease semantics.

All the standard submit flags now apply to gs ints (--fill,
--no-publish, --update-only, etc.), so users can keep the previous
"push the branch only" behavior with --no-publish if desired.

[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.}