Add Exodus workroom demo readiness checker

[mdheller]

Implements Phase 4 of #478.

Scope:

• add tools/check_exodus_workroom_demo.py
• add docs/exodus-workroom-demo-runbook.md

What this provides:

• local cross-repo artifact presence check for the synthetic Exodus Migration Workroom demo
• optional validator execution across exodus, prophet-workspace, and sociosphere
• default checkout paths under ~/dev
• path overrides for non-default checkouts
• explicit synthetic/offline boundary

Non-goals:

• no provider credentials
• no provider API calls
• no provider-side writes
• no destructive operations
• no workspace manifest edits
• no pin/ref movement
• no resolved-lock regeneration
• no production migration claim

Related:

• Exodus Migration Workroom demo v0 #478
• Add synthetic Exodus Migration Workroom demo fixture exodus#18
• Add Exodus Migration Workroom bridge prophet-workspace#20
• Add Exodus workroom demo integration report #480