integration: Add state model for integration branch

[ed-irl]

Introduce repo-scoped integrationInfo carrying a singleton integration
branch name, the upstream branch, the last pushed hash for
--force-with-lease leasing, and an ordered list of tips with the hashes
recorded at the last rebuild.

Adds Store.Integration / Store.SetIntegration as the public API and
bumps the state storage layout to VersionThree. The version is upgraded
only when an integration is configured, so existing repos that never opt
in keep V1 or V2.

SetRemote preserves integration through remote changes; clearing the
integration drops the version back to its prior level.

Extends the spice.Service Store interface with Integration and
SetIntegration so the service layer can read and write the
configuration. spice.Service.RenameBranch now rewrites any integration
tip that references the renamed branch; rename failures of the
integration update are surfaced as warnings rather than aborting the
primary rename.

Also adds Store.PendingIntegrationRebuild,
Store.SetPendingIntegrationRebuild, and
Store.ClearPendingIntegrationRebuild for persisting a rebuild that was
paused mid-flight (typically by a merge conflict). The handler uses
this to support resuming after the user resolves conflicts in the
worktree.

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • docs: Add shared protocol guide for script integrations #1249
    • scriptrun: Generic runner + shared protocol surface #1188
      • integration: Add state model for integration branch #1165 ◀
        • git: Add Merge, IsClean, and CheckoutNewBranch worktree helpers #1172
          • integration: Add handler package for integration branch operations #1173
            • integration: Wire gs integration command tree #1174
              • integration: Auto-rebuild integration branch after restack #1175
                • integration: Regenerate conflicted derived files on merge #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.}

[diegocostares]

I love the idea