Allow archived task IDs to remain reserved during allocation

[adinballew]

Closes #1042.

Summary

• add the opt-in reserve_archived_ids configuration setting, defaulting to false
• include archived task IDs in allocation when the setting is enabled
• report duplicate IDs involving archived tasks as diagnostic-only findings in doctor and the browser warning
• keep archive files outside automatic duplicate repair

Verification

• 84 focused core/CLI tests pass
• 52 focused browser tests pass in isolated processes
• biome check
• TypeScript check
• build
• diff check