integration: tip advance moves tips to upstack leaves

[ed-irl]

Extending a stack above a configured tip leaves the tip "behind" —
the integration branch keeps merging the old tip and ignores the new
top of the stack. The workaround so far has been 'tip remove old'
plus 'tip add new', which gets tedious when several tips drift
together.

Add 'gs integration tip advance' that walks the upstack tree of each
configured tip via graph.Tops, swaps the tip for its leaves, and
dedups across tips. Linear case ('low -> middle -> high' with tip
'low') collapses to a single replacement: 'low' becomes 'high'.
Forked case ('base -> {fork-a, fork-b}' with tip 'base') expands to
both leaves. Already-at-the-top tips are left alone with no log
spam. An empty advance reports "No tips to advance".

The optional positional argument scopes the operation to a subset of
configured tips, so 'tip advance fork-a' walks only that tip.
Unrecognized names abort before any mutation so we never leave the
tip list half-updated.

[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.}