feat: add session persistence to CLI

- Add save-only endpoint (POST /conversations/{id}/save) for persisting
  messages without triggering agent execution
- Add sessions command to list previous conversation sessions
- Add --resume flag to chat command for resuming sessions by ID
- Buffer streamed responses and save after completion
- Update AGENTS.md with session commands and remove outdated limitation
- Add 3 new tests for save endpoint (208 total)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/domains/conversations/router.py

@@ -15,6 +15,8 @@ from src.domains.conversations.schemas import (
     ConversationResponse,
     CreateConversationRequest,
     MessageResponse,
+    SaveMessagesRequest,
+    SaveMessagesResponse,
 )
 from src.domains.conversations.service import ConversationService
 from src.shared.auth import require_auth
@@ -187,3 +189,51 @@ async def add_message(
         total_tokens=conversation.total_tokens if conversation else 0,
         summarized=summarized,
     )
+
+
+@router.post("/{conversation_id}/save", response_model=SaveMessagesResponse)
+@logged()
+async def save_messages(
+    conversation_id: UUID,
+    request: SaveMessagesRequest,
+    session: AsyncSession = Depends(get_session),
+    user=Depends(require_auth),
+) -> SaveMessagesResponse:
+    """
+    Save a user/assistant message pair without triggering agent execution.
+
+    Used by CLI when streaming responses separately via /agents/stream.
+    This allows persisting the exchange after streaming completes.
+    """
+    service = ConversationService(session)
+
+    # Verify conversation exists and user owns it
+    conversation = await service.get(conversation_id)
+    if not conversation:
+        raise HTTPException(status_code=404, detail="Conversation not found")
+
+    if conversation.user_id != user.id:
+        raise HTTPException(status_code=403, detail="Not authorized")
+
+    # Save user message
+    user_message = await service.add_message(
+        conversation_id=conversation_id,
+        role="user",
+        content=request.user_content,
+    )
+
+    # Save assistant message
+    assistant_message = await service.add_message(
+        conversation_id=conversation_id,
+        role="assistant",
+        content=request.assistant_content,
+    )
+
+    # Get updated conversation for total tokens
+    conversation = await service.get(conversation_id)
+
+    return SaveMessagesResponse(
+        user_message=MessageResponse.model_validate(user_message),
+        assistant_message=MessageResponse.model_validate(assistant_message),
+        total_tokens=conversation.total_tokens if conversation else 0,
+    )

webber-api/src/domains/conversations/schemas.py

@@ -21,6 +21,15 @@ class AddMessageRequest(BaseModel):
     content: str = Field(..., min_length=1, description="Message content")
 
 
+class SaveMessagesRequest(BaseModel):
+    """Request to save a message pair without triggering agent execution.
+
+    Used by CLI when streaming responses separately via /agents/stream.
+    """
+    user_content: str = Field(..., min_length=1, description="User message content")
+    assistant_content: str = Field(..., min_length=1, description="Assistant response content")
+
+
 # === Response Schemas ===
 
 class MessageResponse(BaseModel):
@@ -77,3 +86,10 @@ class AddMessageResponse(BaseModel):
         default=False,
         description="Whether context was summarized due to token limit"
     )
+
+
+class SaveMessagesResponse(BaseModel):
+    """Response after saving messages (no agent execution)."""
+    user_message: MessageResponse
+    assistant_message: MessageResponse
+    total_tokens: int