feat(source-control): worktree-create fresh base should resolve the default branch of non-origin remotes

[kyle-sexton]

Summary

The shared worktree helper (plugins/source-control/scripts/worktree-create.sh, added in #399) resolves the fresh base ref by probing refs/remotes/origin/HEAD only. In a repo cloned with a non-origin default remote (e.g. git clone -o upstream), upstream/HEAD may be correctly cached but origin/HEAD is absent, so --base-ref fresh falls back to local HEAD (with a warning) and a worktree created from a feature branch carries unpushed/local commits into a supposedly-fresh worktree.

Not a regression (context)

This is a pre-existing, shared limitation, not something #399 introduced: Claude Code's own native fresh resolution is also origin-centric (per https://code.claude.com/docs/en/worktrees it keeps origin/HEAD current). #399 preserved that same origin-centric behavior for the interactive path and added a loud stderr warning on the HEAD fallback, so the failure is visible rather than silent. This issue tracks closing the gap properly.

Proposed fix

Before falling back to HEAD, resolve the repo's effective default remote generically:

1. the current branch's configured remote (git config branch.<name>.remote), then
2. origin if it exists, then
3. the sole remote if there is exactly one.

Probe refs/remotes/<remote>/HEAD for that remote. Keep the HEAD fallback + warning only when no remote default can be resolved.

Acceptance

• --base-ref fresh in a -o upstream clone bases the worktree on upstream's default branch, not local HEAD.
• Test fixture: a fixture whose only remote is named something other than origin.
• The HEAD fallback + warning remains for the genuinely-remoteless case.

Source

Raised by a Codex review thread on #898 (PR for #399, Phase A).

[kyle-sexton]

This was generated by AI (triage lane, gate-audit pass — cycle 2 retry, instance melo-lap-001-triage-20260727T125559Z).

Gate-audit verdict: LIFT — flipped needs-human → agent-ready.

Resolution order is a fully specified numbered fallback chain (branch-configured remote → origin → sole remote), the edge case is pre-decided (HEAD fallback+warning stays only when no remote resolves), and a 3-item acceptance checklist including a test fixture closes it out.

No human-floor concern applies (not security-surface, not a dependency pin, not cross-cutting, not irreversible). Consensus: fable-lens panel (read-only, prior cycle) + an independent fresh-context opus second lens + this lane's own read all agree LIFT, on an issue text that poses no unresolved design choice.

This lane triages/gate-audits only — it does not execute the fix. Eligible for autonomous pickup.

[kyle-sexton]

[kyle-sexton]

Handoff: PR #1730 implements the three-rung default-remote resolution and closes this issue on merge.