fix: populate Slack notification fields on workflow_dispatch

[cweinberger]

Summary

• Fix null Committer, Message, and Change fields in Slack deployment notifications when triggered via workflow_dispatch (manual trigger)
• Adds a step in component-initialize.yml that fetches HEAD commit info via the GitHub API and writes a push-compatible event file before the slack-notifier-cli runs

Root cause

The slack-notifier-cli binary parses $GITHUB_EVENT_PATH trying push → opened → created event structures. On workflow_dispatch, the event payload has none of these, so all fields fall through to null.

How it works

1. New step runs only on workflow_dispatch events (if: github.event_name == 'workflow_dispatch')
2. Fetches the HEAD commit (${{ github.sha }}) info via gh api
3. Writes a push-compatible JSON structure (with head_commit and pusher) to $GITHUB_EVENT_PATH
4. The slack-notifier-cli then parses this enriched event and populates Committer, Message, and Change correctly

Test plan

• Trigger a manual workflow_dispatch deployment on any service → verify Slack notification shows Committer, Message, and Change
• Verify tag-push deployments still work (this step is skipped via if condition)
• Verify PR-triggered workflows still work

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