fix(ci): use full git history on PRs and fail loudly on diff errors

[jlc488]

Summary

Two bugs in the CI workflow from #1 made path-filtered builds silently skip on every PR. Discovered while opening #2 — the matrix went empty and the build job was reported as "skipping" even though `easy-paging-demo/` was clearly the changed directory.

Two bugs, one symptom

Bug 1 — GitHub Actions ternary trap on `fetch-depth`

```yaml
fetch-depth: ${{ github.event_name == 'pull_request' && 0 || 1 }}
```

The intent was "0 (full history) on PRs, 1 (shallow) otherwise". But in Actions expressions, `0` is falsy, so `0 || 1` short-circuits to `1`. The expression evaluates to `1` on both PRs and main pushes. PR checkouts had no history beyond the merge commit.

Bug 2 — silent failure on `git diff`

```bash
mapfile -t changed_files < <(git diff --name-only "$base" HEAD)
```

With bug 1 in effect, `git diff` exited with `fatal: bad object` because the base SHA was never fetched. But process substitution does not propagate the subshell's exit status under `set -e` — so the script kept going with an empty `changed_files` array. The matrix went empty. The build job was marked as "skipping". CI looked green but had built nothing.

Fix

• `fetch-depth: 0` unconditionally. Cost is negligible for this repo and removes the ternary footgun entirely. A code comment now flags the trap so the next person doesn't reintroduce it.
• Two-stage `git diff` with explicit error check, so `set -e` actually catches a non-zero exit. `::error::` annotation points at the likely cause (shallow checkout) for the next debugger.

Verification

Will be confirmed when #2 is rebased on top of this fix:

• On that PR's CI run, the `detect` job should output `demos: ["easy-paging-demo"]` and `any: true`.
• The `build (easy-paging-demo)` matrix job should actually run `./gradlew build` rather than skipping.

Cannot be verified by this PR's own CI run — this PR doesn't touch any demo directory, so the matrix will (correctly) be empty and `build` will (correctly) skip. The fix only takes effect when there's a demo change to detect.

Test plan

• CI on this PR: `detect` job passes (with the new defensive diff handling), `build` skipped (no demos changed — expected)
• After merge + rebase of feat: add easy-paging-demo (first runnable example) #2: `detect` job emits `easy-paging-demo` in the matrix, `build` job runs end-to-end and turns green