fix(devtools): measure PR size against origin's default branch

[davidmoralesz]

When check-pr-size.sh works the base out for itself and lands on the default branch, it now reads origin/<default> and only falls back to the local branch when origin has none.

Why

The local default branch only moves on a pull. A branch cut from a fresher origin/main had its merge base at the stale local main, so every file main gained in between was counted as the branch's own. Seen on #45: a 2-file change reported as 4 because local main was one merge behind.

What does not change

• A base that was named (--base, PR_BASE) still resolves local-first, so ci-pr and explicit calls behave as before.
• A gh stack parent that is not the trunk still resolves local-first, since a stack's layers are local branches.
• With no origin/<default> ref, the local branch is used as before.

Tested

• bash .devtools/test-hooks.sh: all passed, with two new cases (origin preferred for a worked-out default branch; a named base stays local).
• shellcheck clean on both files.
• In this checkout, with local main one commit behind: the old script reports 3/20 files changed vs main on a branch with no changes, the new one 0/20 files changed vs origin/main.

Known limit

It compares against origin as of the last fetch; it does not fetch. A push from a clone that has not fetched in a while can still overcount, and CI remains the real gate.