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/shell.instructions.md

@@ -0,0 +1,12 @@
+---
+name: "Shell Standards"
+description: "Cross-project shell scripting standards for safety and predictable automation."
+applyTo: "**/*.sh"
+---
+
+- Use defensive scripting with explicit error handling.
+- Quote variables and paths.
+- Validate inputs before using them in commands.
+- Keep scripts modular and avoid hidden side effects.
+- Avoid unsafe deletion patterns, silent failures, and hardcoded environment-specific paths.
+- Keep scripts ShellCheck-compatible where feasible.