Add signal contracts for reachability, exploitability, trust, and unknown symbols

- Introduced `ReachabilityState`, `RuntimeHit`, `ExploitabilitySignal`, `ReachabilitySignal`, `SignalEnvelope`, `SignalType`, `TrustSignal`, and `UnknownSymbolSignal` records to define various signal types and their properties.
- Implemented JSON serialization attributes for proper data interchange.
- Created project files for the new signal contracts library and corresponding test projects.
- Added deterministic test fixtures for micro-interaction testing.
- Included cryptographic keys for secure operations with cosign.

docs/notifications/fixtures/redaction/sample.json

@@ -5,5 +5,5 @@
   "subject": "User signup",
   "body": "User john@example.com joined",
   "redacted_body": "User ***@example.com joined",
-  "pii_hash": "TBD"
+  "pii_hash": "dd4eefc8dded5d6f46c832e959ba0eef95ee8b77f10ac0aae90f7c89ad42906c"
 }