Add repo name prefix to Slack PR completion messages

[cb-nileshrathi]

Summary

• Capture full repo name on PR payloads and include it in Slack notification metadata.
• Prefix bot-merged “completed” Slack messages with the repo name ([owner/repo] prefixes) for clearer context.

[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scanner	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

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