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.

docs/handoff/20260622-handoff-79.md

@@ -7,15 +7,15 @@ Amendment 2), which grew out of a one-line copy-rule fix.
 
 ## §1. State
 
-**Branch `main`.** Commits this session so far (all on `main`, **not pushed**):
-`07575da` (#39 gate) · `e88fa79` (handoff-77 + changelog rule) · `3ad4aff`
-(#36 help) · `64818c0` (handoff-78) · `1a2002d` (**ADR-0005 Amendment 2** —
-the blob-drop decision record).
+**Branch `main`.** Commits this session (`1a2002d` ADR-0005 Am2 + `6b4c4dc`
+blob removal, plus the earlier `07575da` #39 · `e88fa79` handoff-77 · `3ad4aff`
+#36 · `64818c0` handoff-78) — **the user pushed these.** A follow-up
+**`style:` rustfmt commit + this CLAUDE.md/handoff doc commit** land on top
+(see §5) and are the only unpushed work at handoff time.
 
-**Uncommitted: the blob-removal implementation** (39 files) — staged for review,
-commit proposed at session end (see §4). **Full suite green: 2520 passed / 0
-failed / 1 ignored** (1810 lib + 7 e2e_pty + 503 it + 200 typing).
-`clippy --all-targets -D warnings` + `fmt --check` clean.
+**Suite green: 2521 passed / 0 failed / 1 ignored** (1810 lib + 8 e2e_pty + 503
+it + 200 typing), `fmt --check` + `clippy -D warnings` clean — **verified via
+`nix develop -c`** (the CI-exact way; see §5's lesson).
 
 **Closed this session:** #36, #39. **Open:** #37 (clause hints), #38
 (diagnostic route — needs a do/defer/close call), #40 (winget release notes).
@@ -92,16 +92,34 @@ is deleted outright.
 ## §4. Process pins / next
 
 - Commits user-confirmed, no AI attribution, append-only, on `main`; **push is
-  the user's step** (everything this session is unpushed).
+  the user's step.**
 - **`/runda` was run on both the ADR (design) and the implementation.** The
   implementation pass found one real gap — the runtime's migrate-on-open glue
   was untested — closed with the new Tier-4 PTY test. It also verified the
   import path migrates (extract→migrate→rebuild) and the non-migration
   `parse_schema` callers fail-safe.
-- Proposed commit split for the implementation: (1) `feat(types)!` — code +
-  tests + CHANGELOG; (2) `docs` — the CLAUDE.md/requirements/website/ADR-x-ref
-  sweep; (3) `docs(handoff-79)`.
 - Consider a `cargo sweep` at this milestone.
 - **Next open issues:** #37 (clause hints, on-mission), #38 (escalate
-  do/defer/close first), #40 (winget release notes). Plus: blob removal means a
-  `cargo sweep`-worthy build and a good moment to push.
+  do/defer/close first), #40 (winget release notes).
+
+## §5. CI/fmt lesson (cost a red CI run — don't repeat)
+
+The blob commit `6b4c4dc` **failed the CI `fmt` gate** (run 92, `ci.yaml`).
+Two compounding causes, both fixed here:
+
+1. **`fmt` IS gated now** (`cargo fmt --check`, ADR-ci-002 Amendment 1 / issue
+   #35) — but `CLAUDE.md` still said "fmt is intentionally not gated yet." That
+   stale note bred complacency. **Corrected `CLAUDE.md`** (the gate is fmt +
+   clippy + test, run via `nix develop -c`).
+2. **A broken local check masked the failure:** `cargo fmt --check 2>&1 | tail
+   -1 && echo "fmt clean"` tests `tail`'s exit code (always 0), **not** fmt's —
+   so "fmt clean" printed over real diffs. **Always read the exit code**, never
+   a piped `… | tail && echo clean`.
+
+**Going forward, verify the CI way:** `nix develop -c cargo fmt --check` /
+`nix develop -c cargo clippy --all-targets -- -D warnings` / `nix develop -c
+cargo test` — matching the pinned 1.95.0 toolchain *and* the devShell env, and
+checking exit codes. (A bare host `cargo` happened to be 1.95.0 this time, so
+the divergence was the masking pipe, not the toolchain — but `nix develop -c`
+guards both.) The `style:` rustfmt fix + this doc update land on top of
+`6b4c4dc`; CI re-runs green on the next push.