Add base instances for instance config inheritance

Instances can be marked as a base with instance.is_base and other
instances inherit config values from it through instance.base_id.
The controller copies base values into inheriting configs and keeps
track of inherited fields in the hidden instance.inherited_fields, so
hosts receive resolved configs and need no changes. Setting a field
stops inheriting it, `instance config inherit` undoes that.

Bases cannot be assigned to a host, are hidden from `instance list`
unless --bases is given, and cannot be deleted or unmarked while
other instances inherit from them. Chains of bases are allowed and
cycles are rejected.

Part of #200

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/controller/src/InstanceRecord.ts

@@ -83,6 +83,7 @@ export default class InstanceRecord {
 			this.startedAtMs,
 			this.updatedAtMs,
 			this.config.get("instance.exclude_from_start_all"),
+			this.config.get("instance.is_base"),
 		);
 	}