Add weekly resource staleness report agentic workflow

Scans agents, prompts, instructions, hooks, and skills folders to
identify resources that haven't had a major change in over 30 days
(stale) or 14 days (aging). Runs weekly or on demand and files a
GitHub issue with the report.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitattributes

@@ -26,3 +26,5 @@
 *.ico binary
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+.github/workflows/*.lock.yml linguist-generated=true merge=ours