Add .gitbay/ci.yml and make clippy clean #4

There is no CI configuration. The privacy gate is only enforced on a developer's Mac.

cargo clippy --all-targets currently fails: approx_constant at crates/signal-schema/tests/privacy_invariant.rs:43 (Value(3.14_f64)).

Constraints from the sibling repos: the gitbay runner is Linux (see the comment in orgo/.gitbay/ci.yml), so swift test cannot run there. The differential secret-typing tests need zsh with the zsh/zpty module on the runner, and the git collector tests need git with a commit identity.

Done when:

• .gitbay/ci.yml runs cargo test --locked and cargo clippy --all-targets --locked -- -D warnings on every push.
• The clippy error is fixed.
• Either zsh is confirmed present on the runner and the differential tests run in CI, or the job installs it. The tests must not be silently skipped.
• The Swift wire-contract tests run from a pre-push hook on macOS, following orgo's .githooks/pre-push pattern, and the README says so.
• gitbay build list shows a green build for main.

Closing: this repo no longer has CI.

The runner on the host polls with --repos krz/gitbay (the #92 mitigation), and runner next resolves each argument to an exact repo id — no wildcard, no org scoping. Builds queued here were therefore never claimed; 1-9 all ended cancelled.

.gitbay/ci.yml is removed in !4. The checks it ran move to .githooks/pre-push, alongside the Swift tests that were already there: zsh/zpty preflight, cargo test --locked, cargo clippy --all-targets --locked -- -D warnings, swift test.

The clippy half of this issue landed on main already (approx_constant, doc_lazy_continuation); clippy still runs, just from the hook. The "green main build" condition is dropped as unreachable.