fix: scope agent-beads doctor routes to the selected rig

[Bella-Giraffety]

Summary

• gt doctor --rig was still loading route prefixes from other rigs in agent-beads-exist.
• That caused missing-agent reports, and --fix behavior, to include unrelated rigs.
• Scope route-derived rig expectations to the selected rig and add focused Run()/Fix() regression coverage.

Why

Fixes #3046.

agent-beads-exist was still traversing every registered rig route even when doctor was explicitly scoped to one rig. That made a healthy selected rig look broken, and could make --fix do unrelated work outside the requested scope.

Validation

• GOTOOLCHAIN=auto go test ./internal/doctor -run 'TestAgentBeadsExistCheck|TestRoutesCheck_DirectLayoutRig|TestDetermineRigBeadsPath_Containment'

Notes

• This PR intentionally scopes only route-derived rig agents.
• It does not change town-level mayor/deacon handling under --rig.

[Bella-Giraffety]

This PR has been rebuilt on a clean upstream main base and force-pushed so it now contains only the route-scoping fix and focused tests. The previous branch accidentally included unrelated local workspace artifacts and fork-only history.