refactor: reorganize into monorepo with separate subprojects

Structure webber into three independent subprojects:
- webber-api/: FastAPI backend server with all agent code
- webber-cli/: Standalone CLI client (renamed from cli/ to webber_cli/)
- webber-sandbox/: Test project for functional testing

Key changes:
- Each subproject has its own .venv (Python 3.12+)
- Added sandbox.sh for managing test project templates
- Created sandbox-templates/ with calculator-cli and empty starter
- Updated CI/CD for prefixed tags (api/v*, cli/v*)
- Added comprehensive AGENTS.md with operational instructions
- Added gitignore filtering to glob and grep tools
- Created pyproject.toml for each subproject

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

webber-api/src/domains/agents/base.py

@@ -0,0 +1,167 @@
+"""
+Base classes and registry for agent implementations.
+
+All agents are built on PydanticAI and registered in a central registry.
+"""
+from abc import ABC, abstractmethod
+from dataclasses import dataclass, field
+from typing import Any, Protocol, runtime_checkable
+
+from pydantic_ai import Agent
+
+from src.shared.logging import get_logger
+
+logger = get_logger(__name__)
+
+
+@dataclass
+class AgentContext:
+    """
+    Base context passed to all agent tools.
+
+    Subclass this for agent-specific context (e.g., ExploreContext).
+    """
+    working_dir: str
+    allowed_paths: list[str] = field(default_factory=list)
+    timeout_seconds: int = 120
+
+
+@runtime_checkable
+class AgentProtocol(Protocol):
+    """Protocol that all agents must implement."""
+
+    @property
+    def name(self) -> str:
+        """Unique identifier for the agent."""
+        ...
+
+    @property
+    def description(self) -> str:
+        """Human-readable description of what the agent does."""
+        ...
+
+    @property
+    def agent(self) -> Agent:
+        """The underlying PydanticAI agent."""
+        ...
+
+    async def run(self, prompt: str, **kwargs: Any) -> str:
+        """
+        Execute the agent with a prompt.
+
+        Args:
+            prompt: User prompt/query
+            **kwargs: Additional arguments (working_dir, etc.)
+
+        Returns:
+            Agent response as string
+        """
+        ...
+
+
+class BaseAgent(ABC):
+    """
+    Abstract base class for agent implementations.
+
+    Provides common functionality and enforces interface.
+
+    Usage:
+        class ExploreAgent(BaseAgent):
+            name = "explore"
+            description = "Fast codebase exploration"
+
+            def _create_agent(self) -> Agent:
+                # Create and configure PydanticAI agent
+                ...
+
+            async def run(self, prompt: str, **kwargs) -> str:
+                # Execute agent
+                ...
+    """
+
+    @property
+    @abstractmethod
+    def name(self) -> str:
+        """Unique identifier for the agent."""
+        pass
+
+    @property
+    @abstractmethod
+    def description(self) -> str:
+        """Human-readable description."""
+        pass
+
+    @property
+    def agent(self) -> Agent:
+        """Lazy-loaded PydanticAI agent."""
+        if not hasattr(self, '_agent') or self._agent is None:
+            self._agent = self._create_agent()
+        return self._agent
+
+    @abstractmethod
+    def _create_agent(self) -> Agent:
+        """
+        Create and configure the PydanticAI agent.
+
+        Override this to set up model, system prompt, and tools.
+        """
+        pass
+
+    @abstractmethod
+    async def run(self, prompt: str, **kwargs: Any) -> str:
+        """Execute the agent."""
+        pass
+
+
+# === Agent Registry ===
+
+_AGENT_REGISTRY: dict[str, BaseAgent] = {}
+
+
+def register_agent(agent: BaseAgent) -> BaseAgent:
+    """
+    Register an agent in the global registry.
+
+    Args:
+        agent: Agent instance to register
+
+    Returns:
+        The registered agent (for decorator chaining)
+    """
+    if agent.name in _AGENT_REGISTRY:
+        logger.warning(f"Overwriting existing agent: {agent.name}")
+
+    _AGENT_REGISTRY[agent.name] = agent
+    logger.info(f"Registered agent: {agent.name}")
+    return agent
+
+
+def get_agent(name: str) -> BaseAgent | None:
+    """
+    Get an agent by name.
+
+    Args:
+        name: Agent name
+
+    Returns:
+        Agent instance or None if not found
+    """
+    return _AGENT_REGISTRY.get(name)
+
+
+def list_agents() -> list[dict[str, str]]:
+    """
+    List all registered agents.
+
+    Returns:
+        List of agent info dicts with name and description
+    """
+    return [
+        {"name": agent.name, "description": agent.description}
+        for agent in _AGENT_REGISTRY.values()
+    ]
+
+
+def get_registry() -> dict[str, BaseAgent]:
+    """Get the full agent registry."""
+    return _AGENT_REGISTRY.copy()