Fire the release Slack alert from publish_release; warn when the webhook is unset

[PavelMakarchuk]

Problem

The Slack release alert was called from one place — the populace-publish-release CLI. Any promote that reached latest.json another way (a build script calling publish_release() directly, an experimental promote-to-latest) published silently. That's the same abnormal path that shipped recent buildi/buildj builds with no reform_validation.json — one root cause, and the missing Slack alert was the visible symptom.

Worse, when SLACK_WEBHOOK_POPULACE_* was unset, notify_release returned False silently, so a misconfigured environment looked identical to a healthy one.

Change

• Couple the alert to the promotion. Move the notify_release call into publish_release(), right after latest.json is committed, so every publish path announces the release. Gated on update_latest (a --no-latest publish moves no pointer, so it isn't a new release) and a new notify flag (default True; False for tests / dry-runs / re-publishes).
• Make a missing webhook loud. notify_release gains warn_if_unset: when the env var is missing it logs a ::warning:: instead of returning silently, so a release that publishes without an alert shows up in the log. publish_release passes it.
• Remove the now-redundant notify_release call from the publish CLI.

Tests

• slack: warns when unset+requested, stays silent otherwise.
• release: publish_release announces after the pointer with the right id/timestamp; notify=False skips it; autouse fixture keeps publish tests from posting to a real webhook if one is set in the env.
• test_slack / test_release / test_publish_guard all green; ruff clean. (The one failing test in the suite, test_loader::test_live_load…, is a pre-existing live test failing on a local pe-us version mismatch — unrelated.)

Note

This makes silent skips impossible, but the alert still only sends when SLACK_WEBHOOK_POPULACE_US is set wherever the publish runs (the build machine) — put it in tools/release.env or export it. With this change, if it's missing you'll now see a warning in the publish log instead of silence.

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