Add maintain-copilot-history prompt for chat-driven history maintenance

Closes the pattern gap: cleanup, restore, and purge scripts now have a
corresponding prompt.md for conversational invocation. Defaults to dry-run,
requires confirmation before --execute, and surfaces the protected-paths list.

resources/prompts/maintain-copilot-history.prompt.md

@@ -0,0 +1,31 @@
+---
+name: "maintain-copilot-history"
+description: "Quarantine, restore, or purge Copilot chat and session history artifacts using the shared maintenance scripts."
+agent: "agent"
+tools: [read, search, execute]
+argument-hint: "action=<cleanup|restore|purge> audit-dir=<path> [execute=true] [before=<iso>] [keep-latest=<n>] [filter=<text>] [path=<absolute-path>]"
+---
+
+Manage Copilot history artifacts using the shared quarantine-based maintenance
+scripts.
+
+Requirements:
+
+- Default to dry-run for every action unless the user explicitly asks to apply.
+- For `cleanup`: use `resources/scripts/cleanup-copilot-history.sh`; require
+  `--audit-dir` to set the completion boundary.
+- For `restore`: use `resources/scripts/restore-copilot-history.sh`; prefer
+  `--filter` or `--path` to scope the restore rather than restoring everything.
+- For `purge`: use `resources/scripts/purge-copilot-history-quarantine.sh`;
+  always include `--keep-latest 1` unless the user explicitly overrides it.
+- Never pass `--execute` without confirming the planned changes with the user
+  first; show the dry-run output and ask for confirmation.
+- Summarize what was quarantined, restored, or purged after each action.
+
+Protected paths that must never be targeted:
+
+- `~/.copilot-resources`
+- `~/.copilot/*` symlink targets
+- `~/.copilot-resources-state/publish-log.tsv`
+- `~/.copilot-resources-state/project-ports-registry.json`
+- Repository source under `resources/`, `docs/`, and `templates/`