Clean up obsolete placeholder code

internal/contracts/artifact.go

@@ -1,7 +0,0 @@
-package contracts
-
-// ArtifactResult is a placeholder generated artifact contract.
-type ArtifactResult struct {
-	Schema string `json:"schema"`
-	Path   string `json:"path"`
-}

internal/contracts/session.go

@@ -1,7 +0,0 @@
-package contracts
-
-// SessionManifest is a placeholder durable session-run contract.
-type SessionManifest struct {
-	Schema    string `json:"schema"`
-	SessionID string `json:"session_id"`
-}

internal/contracts/transcript.go

@@ -1,19 +0,0 @@
-package contracts
-
-// SpeakerTranscript is a placeholder transcription artifact contract.
-type SpeakerTranscript struct {
-	Schema    string `json:"schema"`
-	SessionID string `json:"session_id"`
-}
-
-// CanonicalTranscript is a placeholder merged transcript contract.
-type CanonicalTranscript struct {
-	Schema    string `json:"schema"`
-	SessionID string `json:"session_id"`
-}
-
-// ProcessedTranscript is a placeholder polished transcript contract.
-type ProcessedTranscript struct {
-	Schema    string `json:"schema"`
-	SessionID string `json:"session_id"`
-}