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/language-and-tooling-defaults.instructions.md

@@ -0,0 +1,16 @@
+---
+name: "Language and Tooling Defaults"
+description: "Cross-project language and file-type guidance for JS, Vue, SCSS, Markdown, JSON, HTML, Shell, YAML, and config files."
+applyTo: "**/*.js,**/*.mjs,**/*.vue,**/*.scss,**/*.md,**/*.json,**/*.html,**/*.sh,**/*.yml,**/*.yaml,**/*.env,**/*.conf"
+---
+
+- Use `javascript.instructions.md` for JavaScript and MJS files.
+- Use `vue.instructions.md` for Vue SFC files.
+- Use `scss.instructions.md` for SCSS stylesheets.
+- Use `markdown.instructions.md` for Markdown docs.
+- Use `json.instructions.md` for JSON data/config files.
+- Use `html.instructions.md` for HTML templates/pages.
+- Use `shell.instructions.md` for shell scripts.
+- Use `yaml.instructions.md` for YAML configs.
+- Use `env-config.instructions.md` for env and conf files.
+- Prefer project-local instructions when they provide more specific guidance.