chore(ci): raise verify-gate timeout 30 → 60 min for lake build (#229)

After #227 reverted TEST-PROOF-LATENCY / TEST-PROOF-ARINC653 verification
steps from sorry-grep to `cd proofs && lake build`, the first PR after
#227 (#228 Mermaid M3) timed out the verify-gate at the 30-minute hard
budget. lake build against Mathlib on a cold elan cache takes 25–35 min;
the runner ran for 26 min before hitting the timeout, then GitHub
killed the orphan `lake` process during job cleanup.

Bumps the verify-gate job's `timeout-minutes` from 30 → 60 to give
`lake build` headroom while the smithy team adds Lake build-artifact
caching across runs. Once that lands, the timeout can drop toward
the original 20m.

The dedicated "Lean proof typecheck (lake build)" workflow has its
own budget for the same step (already empirically green), so this only
affects the rivet verification gate runs.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: avrabe

.github/workflows/verification-gate.yml

@@ -17,7 +17,12 @@ jobs:
   rivet-verification:
     name: Verification Gate (rivet-driven)
     runs-on: [self-hosted, linux, x64, rust-cpu]
-    timeout-minutes: 30
+    # 60m budget: TEST-PROOF-* steps run `cd proofs && lake build`, which
+    # against Mathlib on a cold elan cache takes 25–35 min. Once smithy
+    # adds Lake build-artifact caching across runs, this can drop back
+    # toward 20m. The dedicated "Lean proof typecheck" workflow has its
+    # own budget for the same step.
+    timeout-minutes: 60
     env:
       CARGO_TERM_COLOR: always
       RUSTFLAGS: -D warnings