Gardener: require manual label, keep Dependabot auto-label

[byrichardpowell]

Summary

Changes the gardener Slack notifier so that the devtools-gardener label (which gates Slack posting) must be applied manually for human-opened issues and PRs. Dependabot-opened PRs are still auto-labeled.

Why

The previous flow applied devtools-gardener to every newly opened issue and PR. This repo gets a lot of PR's, many of which don't need to be reviewed by this team. So it was too noisy.

Dependabot PRs are kept auto-labeled because we always want dependency updates to surface for triage, and there's no human to apply the label.

Behavior matrix

Event	Before	After
Human opens issue/PR	Label applied, Slack post	Nothing
Human applies devtools-gardener	Slack post	Slack post
Dependabot opens PR	Label applied, Slack post	Label applied, Slack post

How it works

gardener-notify-event.yml (capture):

• Dropped opened from issues: types — issues now only capture on labeled.
• Kept opened on pull_request_target: but the job if only admits it when the opener is dependabot[bot].
• labeled events still pass through when the label is devtools-gardener.

gardener-notify-slack.yml (downstream workflow_run):

• The auto-label step is scoped to the Dependabot-opened path (action == opened && login == dependabot[bot]) instead of every opened event.
• No duplicate Slack posts: labels applied here via GITHUB_TOKEN don't trigger another labeled run (GitHub's loop-prevention).

The two-workflow pattern is still needed: Dependabot-triggered workflows have read-only GITHUB_TOKEN and no Actions secret access, so labeling and Slack posting happen in the workflow_run follow-up which runs in the default-branch context.

[Copilot]

Pull request overview

Updates the Gardener Slack notification workflows so that Slack posting is gated by a manually applied devtools-gardener label for human-created issues/PRs, while keeping Dependabot PRs automatically labeled (and posted) to ensure dependency updates remain visible.

Changes:

• Stop capturing issues: opened events so human-opened issues no longer auto-post to Slack.
• Gate pull_request_target: opened capture to Dependabot PRs only; keep labeled capture for manual devtools-gardener.
• Scope downstream auto-labeling to only Dependabot-opened PRs in the workflow_run workflow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/gardener-notify-event.yml	Adjusts event capture triggers/conditions to only pass through manual devtools-gardener labels and Dependabot-opened PRs.
.github/workflows/gardener-notify-slack.yml	Limits auto-labeling to Dependabot-opened PRs while continuing to post Slack summaries for captured events.

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