feat(ci): add weekly on-call Slack notification workflow

[jimchamp]

Posts an on-call announcement every Monday and a deployment-lead reminder every Tuesday, @mentioning people from the roster in pm_config.json. Lead/secondary rotate weekly, computed from days-since-Unix-epoch rather than ISO week number so the rotation doesn't glitch at year boundaries -- no stored index needed.

Adds notify_slack.yml, a reusable workflow wrapping slackapi/slack-github-action for posting (and optionally threading replies to) a Slack message, so future workflows can publish to Slack without duplicating that logic.

[Copilot]

Pull request overview

Adds GitHub Actions automation for posting weekly PM on-call and deployment-lead reminders to Slack, backed by a roster in-repo and a reusable Slack publishing workflow to avoid duplicating Slack posting logic across future workflows.

Changes:

• Adds a scheduled workflow to compute a weekly lead/secondary rotation and post Monday/Tuesday reminders to Slack.
• Adds a reusable workflow_call Slack publisher that can post a parent message and optional threaded replies.
• Introduces pm_config.json to define the on-call personnel roster (Slack @mention IDs).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
.github/workflows/pm_on_call_notification.yml	Scheduled Monday/Tuesday workflow that builds a rotation-based message and calls the reusable Slack notifier.
.github/workflows/notify_slack.yml	Reusable workflow wrapping slackapi/slack-github-action to post a parent message and optional threaded replies.
.github/workflows/config/pm_config.json	On-call roster configuration used by the scheduled workflow.

Comments suppressed due to low confidence (3)

.github/workflows/pm_on_call_notification.yml:52

• This file ends with a blank line that contains trailing spaces (line 52). Trailing whitespace tends to trip pre-commit/linters; please remove the spaces so the file ends cleanly.

      message: ${{ needs.build_message.outputs.message }}
    secrets:
      slack_token: ${{ secrets.SLACK_TOKEN }}

.github/workflows/notify_slack.yml:95

• This workflow ends with trailing whitespace on the final blank line (line 95). Removing it avoids trailing-whitespace lint failures.

            text: ${{ toJson(matrix.reply) }}

.github/workflows/config/pm_config.json:19

• This JSON file appears to end with a blank line containing trailing spaces (line 19). Please remove the trailing whitespace so it ends cleanly.

}

[jimchamp]

I intend to move all configurations for project management workflows to this file.