feat(config): Add cloud_projects schema for project-scoped sync (SPEC-20 Phase 1)

Add CloudProjectConfig model and cloud_projects dict to BasicMemoryConfig:
- CloudProjectConfig tracks local_path, last_sync, bisync_initialized
- cloud_projects: dict[str, CloudProjectConfig] in config
- Fix datetime serialization with mode='json' in model_dump()
- Add comprehensive tests for cloud_projects functionality
- Backward compatible: old configs without cloud_projects load correctly

This enables project-scoped sync configuration tracked in config file
rather than filesystem discovery, preventing phantom projects.

Part of SPEC-20 Simplified Project-Scoped Rclone Sync.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/config.py

@@ -3,11 +3,12 @@
 import json
 import os
 from dataclasses import dataclass
+from datetime import datetime
 from pathlib import Path
 from typing import Any, Dict, Literal, Optional, List, Tuple
 
 from loguru import logger
-from pydantic import Field, field_validator
+from pydantic import BaseModel, Field, field_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 import basic_memory
@@ -39,6 +40,26 @@ class ProjectConfig:
         return f"/{generate_permalink(self.name)}"
 
 
+class CloudProjectConfig(BaseModel):
+    """Sync configuration for a cloud project.
+
+    This tracks the local working directory and sync state for a project
+    that is synced with Basic Memory Cloud.
+    """
+
+    local_path: str = Field(
+        description="Local working directory path for this cloud project"
+    )
+    last_sync: Optional[datetime] = Field(
+        default=None,
+        description="Timestamp of last successful sync operation"
+    )
+    bisync_initialized: bool = Field(
+        default=False,
+        description="Whether rclone bisync baseline has been established"
+    )
+
+
 class BasicMemoryConfig(BaseSettings):
     """Pydantic model for Basic Memory global configuration."""
 
@@ -138,6 +159,11 @@ class BasicMemoryConfig(BaseSettings):
         description="Enable cloud mode - all requests go to cloud instead of local (config file value)",
     )
 
+    cloud_projects: Dict[str, CloudProjectConfig] = Field(
+        default_factory=dict,
+        description="Cloud project sync configuration mapping project names to their local paths and sync state",
+    )
+
     @property
     def cloud_mode_enabled(self) -> bool:
         """Check if cloud mode is enabled.
@@ -427,7 +453,9 @@ def get_project_config(project_name: Optional[str] = None) -> ProjectConfig:
 def save_basic_memory_config(file_path: Path, config: BasicMemoryConfig) -> None:
     """Save configuration to file."""
     try:
-        file_path.write_text(json.dumps(config.model_dump(), indent=2))
+        # Use model_dump with mode='json' to serialize datetime objects properly
+        config_dict = config.model_dump(mode='json')
+        file_path.write_text(json.dumps(config_dict, indent=2))
     except Exception as e:  # pragma: no cover
         logger.error(f"Failed to save config: {e}")