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

[JonathanWorks]

Supersedes #1. Squashed to a single clean commit for clean main history after the local-squash vs remote-history collision that this PR fixes (the irony is intentional).

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, 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, plus THIS very PR's own supersession.

What

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), InSync (no-op), Ahead (rebase), Diverges (apply [merge] on_diverged_remote: remote-squash / restack / abort).

Config

[merge]
push_to_origin = true
on_diverged_remote = "remote-squash"
auto_open_pr_if_missing = true

All three fields default off to preserve pre-0.38 behavior exactly.

Invariants preserved

• Zero force-push in any path.
• gh pr merge stays internal to wt (governance rule unchanged for developer CLI).
• Default push_to_origin = false preserves pre-0.38 model.

Verification

• 7 new unit tests pass.
• cargo test --lib 1000/1001 (1 pre-existing mock_commands flake).
• cargo test --bin wt 584/584.
• cargo clippy --all-targets -- -D warnings clean.
• cargo fmt applied.
• pre-commit gates: no-direct-cmd-output now clean (refactored gh helper onto shell_exec::Cmd).

BRW-IJBPWQ. Ready for review + merge.