integration: tip clean prunes subsumed tips

[ed-irl]

integration: tip clean prunes subsumed tips

When two configured tips lie on the same stack, merging the lower
one into the integration branch produces no content that the higher
tip's merge doesn't already produce. Keeping both wastes a merge
per rebuild and inflates the resolver-prompt surface for no benefit.

Add 'gs integration tip clean' (alias 'prune') that walks each tip's
upstack looking for another configured tip. When one is found, the
lower tip is subsumed: remove it and log which tip subsumes it.
With 'low -> middle -> high' all configured, the prune cascades:
'low' is reported as subsumed by 'middle' (the nearest tip up the
stack), and 'middle' as subsumed by 'high'. Both come out and only
'high' is left.

A clean run with nothing to prune reports 'No subsumed tips to
prune' instead of being silent.

The new command is layered at the CLI: it reads tips via the
existing Show, computes subsumption against the branch graph, and
removes via RemoveTip. The integration handler interface stays
unchanged.

integration-tip-clean: Regenerate derived files

[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.}