docs(wiki): prefer on-demand maintenance

[areshand]

Summary

• remove the planned scheduled trace-reading Wiki steward from the architecture
• define organization Wiki maintenance as an explicitly requested, source-backed patch workflow
• keep both the query service and maintenance agent without Wiki-write or trace-read credentials
• replace the background-steward TODO with concrete on-demand workflow acceptance evidence

Validation

• Wiki service tests: 25/25 passed
• git diff --check
• stale background-steward architecture references: none

The full repository harness was stopped after it stalled in a concurrent tmux-based test run; it produced only existing Rust warnings and no failure before termination. This PR changes documentation only.