feat: integrated diagnostics dashboard and unified log aggregation

## Problem

There is no unified logging aggregation. When an agent fails to start, the Hub UI doesn't display stdout/stderr. To figure out why an agent is stuck, the operator must SSH into the host, run `docker logs scion-hub`, and then find a way to attach to the inner agent's `tmux` logs.

This debugging friction exists across all deployment targets — the lack of integrated diagnostics is a platform gap, not an environment-specific issue.

## Proposal

### (f) Integrated Web Log Viewer

Build a centralized "Diagnostics & Logs" dashboard inside the Hub UI. This panel should:
- Stream live, aggregated logs from the Hub server, the runtime broker, and active agent container outputs
- Present in a single, color-coded stream
- Include built-in log level filters (INFO, WARN, ERROR)
- Support agent-level log isolation (view logs for a specific agent)

### Auto-Healing and Verification Checks

The `make doctor` utility or the Hub startup script should verify that the active container image is fully in-sync with the current Dockerfile dependencies. If a binary (like `docker`) is missing from the container, it should raise a clear warning in the Hub UI:

> "Hub image out-of-sync. Rebuild suggested: run 'make rebuild'."

This prevents silent failures where the hub appears running but can't actually manage containers because its own image is stale.

## Related Issues

- #174 — Blank web UI in workstation mode
- #147 — Admin settings lost on pod restart (symptom of no config diagnostics)

Start by mapping the Hub UI, Hub server, runtime broker, active agent containers, and the make doctor or Hub startup path named in the proposal. Define the dashboard's log sources, filters, agent isolation, and image-sync warning behavior; done means these diagnostics are integrated across deployment targets and stale images produce the stated warning.

Written by the indexing model from the issue text.