release: trigger on the main push itself; guard waits for CI on the SHA

The first armed release run failed at the token mint: crates.io
trusted publishing rejects workflow_run-triggered workflows (status
400, by policy). The trigger moves to push-on-main — a supported
event — and the guard's first step polls the CI workflow's run for
the exact pushed SHA, failing unless it concluded successfully, so
the documented invariant (publication never outruns the verification
bar, always at a CI-validated SHA) is preserved under the new
trigger. workflow_run head_sha references become github.sha, which on
a push event is the same commit. The workflow file name and
crates-io environment are unchanged, so the trusted-publisher
configuration still matches.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Merge pull request #3 from a9lim/dev

Dev