integration: short-circuit gs branch checkout for the integration

[ed-irl]

'gs bco ' previously fell into the generic "branch not
tracked" path: in interactive mode the user was prompted to track the
branch (and accepting would then fail with "cannot track integration
branch"), and in non-interactive mode they got a misleading warning
to run 'gs branch track'. Both are noise — the integration is
deliberately untracked and the user typed its name because they want
to switch to it.

Detect the integration branch in the branchCheckoutCmd up front. If
the requested name matches, emit a single targeted warning that the
branch is throwaway and short-circuit ShouldTrack to return false so
the existing CheckoutBranch path falls through to the plain checkout
without prompting.

This makes 'gs bco ' a viable alias for 'gs intco'
without introducing a separate command path or changing the checkout
handler's contract.

[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.}