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/scss.instructions.md

@@ -0,0 +1,13 @@
+---
+name: "SCSS Standards"
+description: "Cross-project SCSS standards for modular styling and maintainable selector design."
+applyTo: "**/*.scss"
+---
+
+- Keep selectors shallow and specificity low.
+- Reuse variables and mixins instead of repeating style constants.
+- Keep style modules focused; avoid monolithic stylesheet files.
+- Avoid deep nesting and `!important` except as a last resort.
+- Remove duplicated selectors and dead style rules during related edits.
+- Prefer design-token reuse before adding new primitives.
+- Validate with project style lint/build commands when available.