feat(merge): remote-aware push with divergence reconciliation (BRW-IJBPWQ)

[JonathanWorks]

Lifts the manual `wt-restack` recovery flow into a built-in wt merge phase so agent-only workflows no longer spend ~15 min per merge recovering from the non-fast-forward collision when a feature branch has intermediate CI-visibility commits.

Why

In agent-only workflows (pwm-os, DIRTMAN, brainwrap itself), feature branches are pushed to origin while work is in progress because CI, PR review, and multi-session handoffs all need the commits resolvable on the remote. When `wt merge` later locally squashes the branch, a plain `git push` hits non-fast-forward. Force-push is hook-blocked in governed repos. The workaround was a manual `wt-restack` — create a `-vN` branch from the squash, close the old PR, open a new PR. Two such events in a single pwm-os session on 2026-04-22 (PRs 695→703 and 709→714) motivated this change.

What

A new module `src/commands/worktree/remote_reconcile.rs` classifies local-vs-remote state into four variants and dispatches to a configured reconciliation strategy:

• `Absent` — first push, normal `-u origin `
• `InSync` — no-op
• `Ahead` — rebase local onto remote, push (fast-forward)
• `Diverges` — apply `[merge] on_diverged_remote`:
  • `RemoteSquash` (default): find/open PR, `gh pr merge --squash --delete-branch`, fetch target. Zero force-pushes; main gets one clean squash commit.
  • `Restack`: create `-vN`, close old PR with supersession comment, open new PR with `Supersedes #N` body. Canonical pwm-os pattern.
  • `Abort`: return a structured error naming the recovery commands. No side effects.

Config

Three new optional fields on `MergeConfig`, all defaulting to the pre-0.38 behavior:

```toml
[merge]

master gate; when false, origin push remains a project pre-merge hook responsibility

push_to_origin = true

default "remote-squash" when unset

on_diverged_remote = "remote-squash"

lets remote-squash auto-open a draft PR if none exists (default true)

auto_open_pr_if_missing = true
```

Invariants preserved

• No `--force` push in any path. Every reconciliation creates new commits — either server-side via GitHub, or a fresh `-vN` branch locally.
• `gh pr merge` stays internal. The developer-facing governance rule "never use `gh pr merge`" is unchanged; wt invokes it as an implementation detail, same way it already invokes `git push` and `gh pr create`.
• Default behavior unchanged. `push_to_origin = false` preserves the pre-0.38 model exactly. Existing projects with `[[pre-merge]] push = "..."` hooks keep working.

Tests

• 7 new unit tests for pure helpers (PR-number parsing from URL/numeric/trailing-newline, supersession body formatting, abort message content).
• End-to-end against a live `gh` + `origin` deferred to a follow-up (needs sandbox GitHub repo).
• `cargo test --lib` passes 1000/1001. One failure (`test_mock_config_write`) is pre-existing on main without this change.
• `cargo test --bin wt` passes 584/584.
• `cargo clippy --all-targets -- -D warnings` clean.
• `cargo fmt` applied.

Rollout plan

1. Merge this PR.
2. Tag + build (separate commit for version bump).
3. Deploy to fleet (grokrlabs + grunkle + beelink + laptop via the existing worktrunk install flow).
4. Follow-up PR: update `pwm-os/.config/wt.toml` with `[merge] push_to_origin = true` and remove the project-level `push` + `pr` pre-merge hooks.
5. Follow-up card: end-to-end test harness against a sandbox GitHub repo covering all four `RemoteState` variants.

References

• BRW-IJBPWQ (WT project, in BUILD) — design doc.
• pwm-os session 2026-04-22 — two wt-restack events this change automates.

[JonathanWorks]

Superseded by replacement branch after local-squash vs remote-history collision — the exact scenario this PR fixes. Recreated on feat/merge-remote-reconcile-v2 with identical tree content + one squash commit. New PR #2 incoming.