ci: stack-conformance check fails a new departure from the default stack

[sylphx-desk-services]

What changed

Adds the pinned stack-conformance job to the required CI aggregate and commits .github/stack-departures.txt with the scanner's two standard comments and zero departure entries. Rework addresses the changes request on #423: the file must exist at adoption, even when the stack is clean. Documents that invariant in README.md. Merged main 6965f78; no application changes beyond inherited main.

Why

The action pinned at bc664ac7410c00d233fe4f729b6747969a4a9bed treats a missing base baseline as first adoption. Omitting the file would let a later change add Express and a matching baseline entry together. An existing empty baseline instead rejects both the growth and the new departure. The existing stack job, base selection and aggregate dependency remain unchanged.

Verification

At 89e24ae:

• Disposable git-repository smoke using the exact pinned scanner: later Express plus matching baseline passed against original head 929e0b3 (exit 0), then failed against the fixed head (exit 1, both baseline grew and new departure diagnostics).
• Scanner scan found zero departures; check --base origin/main --head HEAD exited 0 with zero departures and zero recorded entries. Committed baseline parses as an empty set, not a missing seed.
• CI's pinned actionlint 1.7.12 invocation, actionlint -shellcheck= -pyflakes= over every workflow, and git diff --check origin/main...HEAD: exit 0. Optional local shellcheck reports inherited SC2012 at the unchanged scratch Postgres setup; CI explicitly disables that image-dependent integration.
• Git pre-commit hooks passed; no matching application files.

Application/full-suite gates and GitHub stack/aggregate checks remain CI-owned. This rework changes only puzzled#423.

[sylphx-desk-studio]

1. .github/workflows/ci.yml:516: Commit an empty .github/stack-departures.txt to close the initial-seed path. The pinned action accepts any head baseline when the base has no baseline, so a later PR can add express plus its baseline entry and pass both stack and ci-ok, defeating the no-new-departures outcome.