… add nightly

Introduces .github/workflows/_test_cortex_m_e2e.yml as a local reusable
workflow that wraps pytorch/test-infra/linux_job_v2.yml with typed models
and timeout inputs. Both the existing per-commit trunk job and a new
nightly job now call it with only their matrix entries and timeouts,
eliminating the duplicated ~25-line script body.

The trunk matrix is unchanged ([mv2, mv3]). The new
test-cortex-m-e2e-nightly job covers four additional models on a daily
cadence: ds_cnn (MLPerf Tiny keyword spotting), mobilenet_v1_025 (MLPerf
Tiny Visual Wake Words / person-detection reference), resnet8 (MLPerf
Tiny image classification), and deep_autoencoder (MLPerf Tiny anomaly
detection). Nightly gets a 180-minute timeout (vs. trunk's default of
120) to give the larger models headroom without blocking per-commit
oncall response.

This follows the same pattern as _test_cadence.yml and _link_check.yml
in this repo.

This change was authored with Claude (claude-opus-4-7[1m]).