Surface agent labels in tctl inventory ls and Web UI Inventory page views

**What would you like Teleport to do?** Surface agent labels in the existing inventory views (specifically tctl inventory ls and the Web UI Inventory page). Both surfaces already expose agent hostname, version, and connection state. Labels should be included as well. **What problem does this solve?** Organizations managing large, heterogeneous Teleport deployments across multiple teams, environments, and ownership boundaries often need to correlate agent inventory data back to the teams responsible for those agents. Labels are one of the primary mechanism in Teleport for encoding ownership, environment, and grouping metadata, but they are absent from both current inventory surfaces. A common operational workflow this blocks today: - Query the inventory to identify agents running a specific Teleport version targeted for upgrade - Extract those agents and their labels to determine environment and ownership - Use label metadata to identify the responsible team or owner - Engage those owners to coordinate upgrade sign-off Without native label support in the inventory view, steps 1 and 2 require custom scripting and/or manual correlation which is particularly painful in environments with hundreds or thousands of agents spread across disparate teams. **If a workaround exists, please include it.** The current workarounds are to either use custom scripting or leverage Managed Updates to design per/team upgrade strategies to side-step the need for manual correlation in the first place (this is not possible in all environments).