fix: stop tracking chat title in audit logs (#24564)

Chat titles can contain sensitive information (secrets, internal project
names, etc.) and should not be visible in audit logs.

- Use truncated chat UUID (first 8 chars) as `resource_target` instead
of the title
- Mark the `title` field as `ActionSecret` so diffs render as `••••••••`

<details><summary>Implementation notes</summary>

Two changes:
1. `coderd/audit/request.go`: `ResourceTarget` for Chat returns
`typed.ID.String()[:8]` instead of `typed.Title`
2. `enterprise/audit/table.go`: Chat `title` field tracking changed from
`ActionTrack` to `ActionSecret`

No frontend changes needed. The frontend already handles `secret: true`
fields.

</details>

> 🤖

enterprise/audit/table.go

@@ -386,7 +386,7 @@ var auditableResourcesTypes = map[any]map[string]Action{
 		"workspace_id":          ActionTrack,
 		"build_id":              ActionIgnore, // Internal lifecycle.
 		"agent_id":              ActionIgnore, // Internal lifecycle.
-		"title":                 ActionTrack,
+		"title":                 ActionSecret,
 		"status":                ActionIgnore, // Churns every message.
 		"worker_id":             ActionIgnore, // Internal.
 		"started_at":            ActionIgnore,