* ci: skip Claude-backed test jobs on fork PRs
Jobs that run the action against the Claude API authenticate via workload
identity federation, which fork PRs cannot mint an OIDC token for, so they
always failed on external contributions. Gate each such job on the PR head
repo matching the base repo; push and workflow_dispatch runs are unaffected.
No-Verification-Needed: CI workflow config only, exercised by Actions on the PR
* test: pin the bare remote's initial branch in fetch-depth test
The shallow-clone case created its bare remote with a plain git init, so
HEAD pointed at whatever init.defaultBranch resolves to (master on CI)
while the test only pushed main. git clone --depth=1 implies
--single-branch, and with a dangling remote HEAD it produces an empty,
non-shallow clone, so the is-shallow assertion failed on runners whose
default branch is not main.
No-Verification-Needed: test-only change