ci: fix Linux runner test failures (shallow clone, git identity, node-pty prebuild)

- checkout with fetch-depth: 0: the git log paging test needs the full
  repository history (default shallow clone yields one commit)
- configure a global git identity on the runner: src/git.ts deliberately
  never sets one and revert/cherry-pick rely on ambient config
- tests/unit.spec.ts: node-pty@1.1.0 ships a prebuilt spawn-helper only for
  darwin/win32; on linux node-gyp compiles it into build/Release. Align the
  assertion with ensureSpawnHelper's candidate list instead of hardcoding
  the prebuilds/ path (the Linux test run was red for this reason)

.github/workflows/ci.yml

@@ -11,6 +11,9 @@ jobs:
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@v4
+        with:
+          # Tests run git log against this repository and need the full history.
+          fetch-depth: 0
       - uses: pnpm/action-setup@v4
         with:
           version: 11
@@ -18,6 +21,13 @@ jobs:
         with:
           node-version: 22
           cache: pnpm
+      # The plugin's git revert/cherry-pick spawn git without setting an
+      # identity (deliberate, see tests/smoke.spec.ts) and rely on ambient
+      # config; the runner has none by default.
+      - name: Configure git identity for tests
+        run: |
+          git config --global user.name "dsh-better-sidebar-ci"
+          git config --global user.email "ci@dsh.invalid"
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
       - name: Typecheck