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/requirements.txt

@@ -0,0 +1,27 @@
+# Webber Production Dependencies
+# Minor version pinning (~=) allows patch updates for security fixes
+# CVE check date: 2026-01-09
+# CVE check sources: PyPI, GitHub Advisories, Snyk, NVD
+
+# Core FastAPI
+fastapi~=0.128.0
+starlette~=0.50.0                      # CVE-2025-62727, CVE-2025-54121 fixed
+uvicorn[standard]~=0.40.0
+pydantic~=2.12.4                       # CVE-2024-3772 (ReDoS) fixed in 2.4.0+
+pydantic-settings~=2.12.0
+
+# Agent Framework
+pydantic-ai~=1.40.0
+
+# HTTP client
+httpx~=0.28.1
+aiofiles~=25.1.0
+
+# CLI
+typer~=0.15.0
+rich~=13.9.0
+
+# Utilities
+python-multipart~=0.0.21
+python-dotenv~=1.2.1
+pathspec~=0.12.1                       # Gitignore pattern matching