ci: add PR quality gates (fmt/clippy/test/build) [#230]

[MichaelTaylor3d]

What

Adds .github/workflows/ci.yml — the professional PR-gated CI for this Rust workspace, triggered on: pull_request (to main) and push to main.

Jobs (stable name: = required-check contexts):

• Rustfmt — cargo fmt --all -- --check
• Clippy — cargo clippy --workspace --all-targets -- -D warnings
• Test + coverage — cargo llvm-cov --workspace --summary-only (runs the whole workspace test suite AND measures line coverage in one instrumented build)

Notes

• No wasm build step needed: the digstore guest wasm is vendored (vendor/digstore_guest.wasm) and wired via .cargo/config.toml DIGSTORE_GUEST_WASM, so the checkout already carries the artifact (CLAUDE.md §3.5).
• Coverage is measure-only (non-blocking) for now — the ≥80% floor is tracked separately (#157). Raising the gate is a one-line change (--fail-under-lines 80).
• Verified locally: fmt clean, clippy clean with -D warnings, 71 tests pass.

Version

Patch bump dig-node-service 0.4.0 → 0.4.1 (chore/ci, no behaviour change) to satisfy the version-increment gate.