docs: correct the stale fmt-gate note in CLAUDE.md; handoff-79

CLAUDE.md said "fmt is intentionally not gated yet" — stale; the gate is
now fmt + clippy + test (ADR-ci-002 Amendment 1 / issue #35), run via
`nix develop -c`. Add CI-exact local-verification guidance and flag the
exit-code-vs-piped-output trap that masked the fmt failure this session.
handoff-79 records the blob removal + the CI/fmt lesson.

CLAUDE.md

@@ -115,15 +115,23 @@ Current decisions at a glance (each backed by an ADR):
   a self-hosted
   **Gitea Actions** pipeline built on a **nix flake** (pinned Rust
   `1.95.0` — one source of toolchain for dev *and* CI) plus a
-  prebuilt CI image. **Gate** (`ci.yaml`): `clippy -D warnings` +
-  `cargo test` on every branch push / PR. **Release** on a `v*` tag
+  prebuilt CI image. **Gate** (`ci.yaml`, every step run via `nix
+  develop -c` so dev and CI share the pinned toolchain): `cargo fmt
+  --check` + `clippy -D warnings` + `cargo test --no-fail-fast` on
+  every branch push / PR. **Verify locally the CI way before pushing**
+  — `nix develop -c cargo fmt --check`, `nix develop -c cargo clippy
+  --all-targets -- -D warnings`, `nix develop -c cargo test` — and read
+  the **exit code**, not piped output (a `cargo fmt --check | tail &&
+  echo clean` masks the real result; a bare host `cargo` may also differ
+  from the pinned toolchain). **Release** on a `v*` tag
   (`release.yaml`): the four non-macOS **D1** targets cross-built
   with `cargo-zigbuild` (Linux musl static + standalone Windows
   `.exe`); the two macOS targets via the **dispatched**
   `release-macos.yaml` on a Tart Apple-Silicon runner (de-nix the
   `libiconv` load path + ad-hoc re-sign). All published to a Gitea
-  release with `.sha256`s. **`fmt` is intentionally not gated yet**
-  (the tree isn't stock-`rustfmt`-clean). Now that this is on `main`,
+  release with `.sha256`s. **`fmt` is gated** (`cargo fmt --check`,
+  stock defaults — enabled once the tree was reformatted on `main`,
+  ADR-ci-002 Amendment 1 / issue #35). Now that this is on `main`,
   `release-macos` is dispatchable (`workflow_dispatch` is
   Gitea-default-branch-only) — **dispatched and verified working**: the
   macOS build + de-nix/re-sign + upload runs end-to-end and the binaries