Harden sidecar setup and read-only team dispatch

[rotwurstesser]

Closes #288

Summary

• Manage one Python environment for all optional sidecars.
• Reject companion tool approvals outside agent mode.
• Dispatch team text prompts through exec in isolated output directories.
• Select an available tribunal judge when Cesar is unavailable.

Verification

• npm run kern:compile
• npm run typecheck
• npm run build
• npm test
• npm pack --dry-run -w packages/dedup --json
• Live agon doctor
• Focused sidecar, companion, team-routing, and Forge integration tests
• Agon multi-engine review