v0.15.1 — the new code's own bugs

Three defects in 0.15.0's own new code, found by re-auditing it the way this project audits everything else. The fix is the most likely place for the next bug — and a new file is a new place for an old one.

Fixed

• A corrupt number in the usage journal or the server registry crashed the command you run to diagnose it. float("abc") raises ValueError, which is neither a json.JSONDecodeError nor an OSError, so it walked straight past the per-line handler in usage.load and the sort key in servers.live_servers: one hand-edited or torn-written character turned agent-dispatch stats --days 7 and agent-dispatch doctor into a traceback. Same escape route as the UnicodeDecodeError-is-a-ValueError round in 0.12.1. All three sites now coerce through one shared usage.as_float — an undatable record falls outside the window, a registry entry with a mangled timestamp sorts last, and nothing raises.
• stats --json printed prose instead of JSON when the journal was empty. That is precisely the state of a fresh install, and therefore the first thing a script piping the output would hit. It now always emits the report object.
• Doc drift: AGENTS.md carried a stale test count.

714 tests (was 708), clean-venv install and twine check verified before tagging.