Integration branch: a merged preview of multiple stacks

[ed-irl-codebot]

Problem

When work is split across several parallel stacks, there is no single
ref that combines them for local testing or preview. Developers
manually merge tips together to get an integrated view, then have to
redo it by hand every time any stack changes.

Proposal

Add an integration branch feature: a managed, throwaway ref that
merges a configured, ordered set of stack tips onto trunk. It is a
derived artifact — rebuilt from its configured tips on demand — giving
one combined branch for local testing without being load-bearing state.

Components:

• State model: repo-scoped integration info carrying the singleton
  integration branch name, upstream branch, last-pushed hash (for
  --force-with-lease leasing), and an ordered list of tips with the
  hashes recorded at the last rebuild. Includes persistence of a paused
  rebuild so a merge conflict can be resolved and resumed.
• Git helpers: Merge, IsClean, and CheckoutNewBranch worktree
  primitives needed to build the merged branch.
• Handler package: orchestration for rebuild/advance/clean and
  conflict-resume logic.
• CLI: the gs integration command tree.
• Hooks: auto-rebuild and auto-submit integration on relevant
  operations.
• Conflict handling: regenerate conflicted derived files during the
  rebuild merge rather than surfacing them as raw conflicts.

Tracked by

• worktree: Add worktree commands, per-worktree trunks, and worktree-aware filtering #1165 — state model for the integration branch
• feat: Add worktree commands, per-worktree trunks, and worktree filters #1172 — Merge, IsClean, CheckoutNewBranch git helpers
• worktree: Add worktree management commands, per-worktree trunks, and worktree-aware filtering #1173 — handler package for integration operations
• integration: Wire auto-rebuild hook on restack commands #1174 — wire the gs integration command tree
• integration: Auto-rebuild integration branch after restack #1175 — auto-rebuild and auto-submit hooks
• integration: Auto-rebuild integration branch on restack #1176 — regenerate conflicted derived files on merge

[abhinav]

I really like the idea of this feature. I've felt the need for it repeatedly. Thanks for the stack. Will work through it after comments.