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/explore/__init__.py

@@ -0,0 +1,16 @@
+"""
+Explore Agent - Fast codebase exploration.
+"""
+from src.domains.agents.explore.agent import (
+    ExploreAgentImpl,
+    ExploreContext,
+    explore_agent,
+    explore,
+)
+
+__all__ = [
+    "ExploreAgentImpl",
+    "ExploreContext",
+    "explore_agent",
+    "explore",
+]