Add runtime contract compatibility policy

[wms2537]

Closes #50

What changed

• extends authenticated contract discovery with lifecycle, feature, replacement, and deprecation metadata
• publishes an explicit minimum 90-day, explicit-sunset-only migration policy for run-as-session/v1
• adds executable current-only, ManagedSession-available, and unsupported compatibility fixtures
• adds a standalone Kertas-shaped contract client that feature-detects the public HTTP surface without runtime-internal imports

Why

ManagedSession storage must not land until clients can distinguish the temporary Run-as-session compatibility mode from real target support and can migrate without database access.

Validation

• npm run typecheck
• 7 focused contract/client tests
• npm run release:gate — PASS at b3b0a32c5e988f9b43f1e69363af6b2bc2492216
• independent Standards review — PASS
• independent Spec review — PASS