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>

sandbox-templates/empty/TASKS.md

@@ -0,0 +1,18 @@
+# My Project - Tasks
+
+A blank starter template. Define your own tasks here.
+
+## Setup
+
+```bash
+python3.12 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+```
+
+## Tasks
+
+- [ ] Define your project goals
+- [ ] Add source files to `src/myproject/`
+- [ ] Add tests to `tests/`
+- [ ] Update `requirements.txt` with dependencies