Stage Reset Bot: include author/triggerer name in Slack message

[linear]

Background

The "Stage Reset Bot" posts to Slack when a stage branch reset completes, but the message has no field identifying who initiated the reset. This makes it hard to follow up — if a reset failed or behaved unexpectedly, there's no clear owner to ping.

Current message shape

Stage Reset Complete — 2026-05-04
1 PRs merged (1 clean, 0 auto-resolved) • 0 failed • 0 missing
All PRs merged successfully. Stage deploy will trigger automatically. • See thread for details.

Thread reply lists the PRs but still no actor.

Desired

Include the name (and ideally a Slack handle / GitHub login) of the user who triggered the reset, e.g. "Triggered by @jaxs" near the top of the message or in the thread.

Acceptance criteria

• The Slack post includes the human-readable name of the user who initiated the reset.
• If the trigger is automated (cron / scheduled run), label it as "Triggered by automation" rather than leaving the field blank.
• Format works for both successful and failed reset messages.

Implementation pointers

• Locate the Stage Reset Bot source in the JesusFilm/core repo (search for the Slack message strings "Stage Reset Complete" and "PRs merged" to pin it down).
• Identify where the reset is triggered (CLI command, GitHub Action, slash command) and surface the actor name from that context — GITHUB_ACTOR, SLACK_USER_ID, or whatever the trigger surface provides.
• Pass it through to the Slack message formatter.

[linear]

ENG-3677