toolchain: default-branch detection — non-origin clone with unpushed branch falls back to possibly-absent origin

[kyle-sexton]

Category: bug (deferred edge, tracked follow-up for PR #467) · Source: pre-merge digest — the edge was validly deferred in-thread but had no durable issue, so the trigger would not survive the merge.

Edge

When branch.<current>.remote is unset and origin/HEAD is absent (non-origin clone, unpushed feature branch), detection forces REMOTE=origin, which may not exist. Degrades gracefully today ("branch diff unavailable" — visible, not silently wrong), common case unaffected.

Fix direction (from the PR thread)

Select a present remote instead of assuming origin; consider adopting git-tree-reset.sh's authoritative fallback (gh repo view --json defaultBranchRef, L109-113) as a SHARED default-branch helper — the same shape #436 (review: origin/main baked across ~7 surfaces) and #442 (source-control origin hardcoded) need. Fixing the class once beats three local fixes.

Related

PR #467 (in-thread deferral record) · #411 (source issue) · #436 · #442

[kyle-sexton]

This was generated by AI during triage. · Category: bug (deferred edge) · Priority: priority: high (toolchain is in the north-star set). Self-labeled severity: low (degrades visibly, common case unaffected) — noted for sub-sort. Routed status: ready.

Plan

Default-branch/remote detection must select a present remote instead of forcing REMOTE=origin when branch.<current>.remote is unset and origin/HEAD is absent (non-origin clone, unpushed feature branch).

Fix the class once, not locally. This issue, #436 (review: origin/main baked across ~7 surfaces) and #442 (source-control origin hardcoded) all need the same thing: a shared default-branch helper whose authoritative fallback is gh repo view --json defaultBranchRef (the shape git-tree-reset.sh already uses). Implement that helper once and route toolchain's detection through it.

Acceptance criteria

• With no upstream and no origin/HEAD, detection resolves the actual default branch via the shared helper (present remote / gh repo view), not an assumed origin.
• The common case (upstream set) is unchanged.
• toolchain, review: origin/main baked as default-branch fallback across ~7 surfaces #436's surfaces, and source-control: remote name origin hardcoded in pull-request create flow (low severity) #442 consume ONE helper — no per-surface origin assumption remains.

Decision defaulted: extract + adopt a shared default-branch helper (authoritative gh repo view --json defaultBranchRef fallback) fixing the #436/#442/#483 class together, rather than a toolchain-local origin patch — veto before merge