feat: structured JSON logging via `VOUCH_LOG_FORMAT=json`

[plind-junior]

What you're trying to do

Running vouch as a service, I can't easily parse its logs. VOUCH_LOG_FORMAT is
referenced in ROADMAP and adapters/generic-mcp/README.md but isn't
implemented anywhere in src/.

Acceptance

• VOUCH_LOG_FORMAT=json emits one JSON object per log line (level, event,
  actor, object_ids).
• Default (unset) keeps the current human-readable format.
• No behavior change beyond formatting.