fix(code-review): base local branch/file diff on origin/<default>, not frozen local ref

[parkerbyrd-ux]

Summary

Local /code-review:start / :deep with no PR (the default branch-scope path) computed its changeset against the bare local default branch via a 3-dot diff (main...HEAD).

In a linked git worktree (e.g. Conductor workspaces) the local main ref is frozen at workspace-creation time and never auto-updates — the live base is the remote-tracking ref origin/main. With 3-dot semantics the merge-base becomes the workspace's creation point, so the "diff" silently balloons to include every PR merged to main since the workspace was created. Observed in the wild: local main 198 commits behind origin/main → git diff main...HEAD = 633 files (operator's ~57 + 198 unrelated merged PRs) vs git diff origin/main...HEAD = ~57 files. The oversized diff flagged findings in unrelated already-merged code and tripped a spurious HIGH coverage-gap.

resolve-scope now, for local branch/file scope:

• detects the repo default branch via origin/HEAD (no longer hardcodes main, fixing a latent master/develop mis-base),
• bases the diff on the live remote-tracking ref origin/<default> (best-effort git fetch; falls back to the local ref when offline / no remote),
• prints a stderr ⚠️ local '<branch>' is N commits behind origin/<branch> warning when the local default has drifted.

base_ref stays the plain branch name, preserving the <BASE_REF> token and STATE_KEY = "<review_branch>:<base_ref>". Base resolution runs only when the diff is actually based on the local default — never on the PR-detected or staged paths, so neither a needless fetch nor a misleading drift warning occurs there.

Feature Flags

None.

Breaking changes

None. The PR path, --base <ref> override, staged (--cached), and GitHub mode are unchanged. Local branch/file reviews now diff against origin/<default>...HEAD instead of <default>...HEAD — this is the intended fix (a stale local default branch no longer inflates the diff).

Test plan

• uv run pytest plugins/ → 1994 passed, 3 skipped
• uv run ruff check . → clean
• uv run pyright → 0 errors
• Updated existing TestResolveScope cases (now assert origin/main...HEAD; made the local-path tests hermetic) and added coverage: offline fallback to local ref, non-main default branch, file-paths pathspec preservation, and the drift-warning helper (fires when behind, silent at 0). PR-path / --base override / worktree-isolation tests unchanged.
• Plugin version bumped 3.1.0 → 3.1.1; commands/start.md usage comment and resolve-scope section updated.

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