feat: implement CreateGrant handler and grant-aware approval engine

- GlobalStreamHandler.handleCreateGrant validates scope (SESSION/STAGE)
  and sends ProtocolError to client on validation failure instead of
  silent log.warn + drop.
- Derive event stageId directly from GrantScope type, removing
  redundant stageIdForEvent tuple.
- SessionOrchestrator integrates ApprovalEngine to check active grants
  before requesting user approval for T2+ tool calls.
- ContextEntry gains EntryRole (SYSTEM/USER/ASSISTANT/TOOL) field for
  proper chat message role mapping.
- RouterContextBuilder.build is now suspend; uses Tokenizer for
  accurate token estimation with fallback to character-based estimate.
- LlamaCppInferenceProvider maps EntryRole to ChatMessage role instead
  of heuristic layer-based inference.

core/context/src/main/kotlin/com/correx/core/context/model/ContextEntry.kt

@@ -3,6 +3,8 @@ package com.correx.core.context.model
 import com.correx.core.events.types.ContextEntryId
 import kotlinx.serialization.Serializable
 
+enum class EntryRole { SYSTEM, USER, ASSISTANT, TOOL }
+
 @Serializable
 data class ContextEntry(
     val id: ContextEntryId,
@@ -10,5 +12,6 @@ data class ContextEntry(
     val content: String,
     val sourceType: String,
     val sourceId: String,
-    val tokenEstimate: Int
+    val tokenEstimate: Int,
+    val role: EntryRole = EntryRole.USER,
 )