Implement experiments.notify notification routing in daily experiment reporting

[Copilot]

The daily experiment report needed to honor experiments.notify destinations when an experiment becomes ready/significant and ensure the experiment owner is explicitly mentioned. This update aligns report instructions with that contract for both issue- and discussion-based notifications.

• Notification routing (experiments.notify)

  • Updated report instructions to read and use notify.issue / notify.discussion from experiment config.
  • Defined deterministic routing order and single-target behavior when both are present.

• Owner mention requirement

  • Added a required owner mention in all notification message templates (ready, significance, guardrail).
  • Standardized owner identity as repository owner for this workflow context.

• Workflow prompt contract updates

  • Extended the “frontmatter fields to read” list to include notify targets and their purpose.
  • Enabled mentions in safe-output configuration so required @owner mentions can be emitted.

Use the `add-comment` safe-output tool to post comments. Select the target in this order:
1) `notify.issue` (when present), 2) `notify.discussion` (when present), 3) skip.
If both are present, post to `notify.issue` only so each notification is emitted once.