fix(ci): use pull_request_target for Dependabot secret access (#137)
Dependabot PRs don't receive repo secrets under pull_request events,
causing DEPLOY_PAT to be empty. Switch to pull_request_target which
runs in the base branch context with full secret access. Safe because
the workflow only runs for dependabot[bot] and only executes pnpm
install -- no PR-supplied code is run.
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