integration: surface integration branch and tips in gs ll

[ed-irl]

The integration branch was previously invisible to the log commands:
users could only learn about it via 'gs integration show'. That made
it easy to forget which tips were participating, or to overlook the
integration entirely when scanning the stack with 'gs ll'.

Show the integration as a standalone top row of 'gs ls' and 'gs ll'
(rendered with a magenta '◆' marker and a "[integration: N tips]"
annotation), and mark each configured tip in the normal tree with a
trailing "[integration-tip]" badge so the relationship reads at a
glance.

The integration item is wired into the list handler as a synthetic
BranchItem with no base and no aboves, then exposed as a separate
root in the branchtree graph so it renders before the trunk tree.
The JSON output gains an "integration" record on the integration
row and an "integrationTip" boolean on tip rows for programmatic
consumers.

No tree edges are drawn between the integration row and its tips:
that fan-in does not fit cleanly into the existing top-down layout
and the markers already convey the relationship clearly enough.

[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.}