Add shared instruction files and copilot history maintenance scripts

- Add 12 cross-project instruction files to resources/instructions/
- Add cleanup-copilot-history.mjs/.sh: quarantine-based history cleanup with dry-run and aggressive scope
- Add restore-copilot-history.mjs/.sh: restore from quarantine by run, path, or filter
- Add purge-copilot-history-quarantine.mjs/.sh: purge old quarantine runs with keep-latest guard
- Add copilot-history-maintenance.test.mjs: 3 passing unit tests covering all three scripts

resources/instructions/markdown.instructions.md

@@ -0,0 +1,12 @@
+---
+name: "Markdown Standards"
+description: "Cross-project Markdown standards for concise, scan-friendly documentation."
+applyTo: "**/*.md"
+---
+
+- Keep documents concise, structured, and easy to scan.
+- Use consistent heading hierarchy.
+- Prefer bullet lists and tables for structured content.
+- Avoid large unbroken paragraphs.
+- Use examples where they improve clarity.
+- Keep formatting markdownlint-compatible when possible.