test: update test suite for OmegaConf config system

- Add tests/conftest.py with make_settings() helper that maps legacy config
  parameter names to new nested OmegaConf structure
- Update all test files to use the conftest fixture
- All 73 tests now pass with new config system
- Maintains backward compatibility via Settings class properties

Also add AGENTS.md with comprehensive AI agent guidelines:
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- Debugging guide for agents working on the project

MIGRATION_NOTES.md

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+# Test Settings Migration Notes
+
+## Overview
+Updated all test files to work with the new OmegaConf-based configuration system while maintaining backward compatibility through a helper function.
+
+## What Changed
+
+### 1. Created `tests/conftest.py` (new file)
+- Implements `make_settings(**kwargs)` helper function
+- Maps legacy parameter names to the new nested structure
+- Used as a shared utility across all test files
+
+### 2. Updated Test Files
+Updated the following test files to use the new helper:
+- `tests/test_bot.py`
+- `tests/test_llm_client.py`
+- `tests/test_proposals.py`
+- `tests/test_thread_memory.py`
+- `tests/test_tools.py`
+
+Each file now:
+1. Imports `make_settings` from `tests.conftest`
+2. Keeps local `_make_settings()` wrapper for compatibility
+3. Calls the shared `make_settings()` function internally
+
+## Parameter Mapping
+
+The `make_settings()` function handles the following legacy-to-new mappings:
+
+### Telegram Config
+- `telegram_bot_token` → `telegram.bot_token`
+- `telegram_allowed_user_ids` → `telegram.allowed_user_ids`
+- `telegram_group_ids` → `telegram.group_ids`
+
+### OpenAI Config
+- `openai_api_key` → `openai.api_key`
+- `openai_base_url` → `openai.base_url` (default: `"https://api.openai.com/v1"`)
+- `openai_model` → `openai.model` (default: `"gpt-4o"`)
+- `openai_system_prompt` → `openai.system_prompt` (uses default if not provided)
+
+### Analysis Config
+- `analysis_target_url` → `analysis.target_url`
+- `analysis_target_api_key` → `analysis.target_api_key`
+- `analysis_cron_hour` → `analysis.cron_hour` (default: `8`)
+- `analysis_cron_minute` → `analysis.cron_minute` (default: `0`)
+
+### Memory Config
+- `thread_memory_path` → `memory.thread_memory_path` (default: `"thread_memory.json"`)
+
+### Tools Config
+- `mcp_server_url` → `tools.mcp_server_url`
+- `mcp_server_api_key` → `tools.mcp_server_api_key`
+
+## Usage Examples
+
+### Before (would fail with new Settings class)
+```python
+settings = Settings(
+    telegram_bot_token="token",
+    openai_api_key="key",
+    telegram_allowed_user_ids=[1, 2, 3]
+)
+```
+
+### After (with new helper)
+```python
+from tests.conftest import make_settings
+
+settings = make_settings(
+    telegram_bot_token="token",
+    openai_api_key="key",
+    telegram_allowed_user_ids=[1, 2, 3]
+)
+
+# Now settings.telegram.bot_token == "token"
+# And settings.telegram.allowed_user_ids == [1, 2, 3]
+# And backward-compat properties still work:
+# settings.telegram_bot_token == "token"
+# settings.telegram_allowed_user_ids == [1, 2, 3]
+```
+
+## How Test Files Use It
+
+Each test file has a local `_make_settings()` wrapper that:
+1. Defines default values specific to that test module
+2. Merges any provided kwargs
+3. Delegates to the shared `make_settings()` function
+
+Example from `test_bot.py`:
+```python
+def _make_settings(**kwargs) -> Settings:
+    """Wrapper for test compatibility."""
+    defaults = dict(
+        telegram_bot_token="test-token",
+        openai_api_key="test-key",
+    )
+    defaults.update(kwargs)
+    return make_settings(**defaults)
+```
+
+This allows tests to override defaults while reusing the mapping logic:
+```python
+# Uses defaults
+settings = _make_settings()
+
+# Override specific values
+settings = _make_settings(telegram_allowed_user_ids=[1, 2, 3])
+```
+
+## Error Handling
+
+The `make_settings()` function will raise a `TypeError` if any unexpected keyword arguments are passed:
+```python
+make_settings(invalid_param="value")
+# Raises: TypeError: Unexpected keyword arguments: invalid_param
+```
+
+This helps catch typos and unexpected parameters early in tests.
+
+## Backward Compatibility
+
+The Settings class retains backward-compatible properties:
+- `settings.telegram_bot_token` → returns `settings.telegram.bot_token`
+- `settings.openai_api_key` → returns `settings.openai.api_key`
+- etc.
+
+This means existing code that reads from Settings using the old property names continues to work.
+
+## Files Modified
+
+1. **`tests/conftest.py`** (NEW)
+   - 91 lines
+   - Core mapping logic for legacy-to-new Settings structure
+
+2. **`tests/test_bot.py`**
+   - Added import: `from tests.conftest import make_settings`
+   - Updated `_make_settings()` to call shared function
+
+3. **`tests/test_llm_client.py`**
+   - Added import: `from tests.conftest import make_settings`
+   - Updated `_make_settings()` to call shared function
+
+4. **`tests/test_proposals.py`**
+   - Added import: `from tests.conftest import make_settings`
+   - Updated `_make_settings()` to call shared function
+
+5. **`tests/test_thread_memory.py`**
+   - Added import: `from tests.conftest import make_settings`
+   - Updated `_make_settings()` to call shared function
+
+6. **`tests/test_tools.py`**
+   - Added import: `from tests.conftest import make_settings`
+   - Updated `_make_llm_settings()` to call shared function
+
+## Testing
+
+To verify the changes work:
+```bash
+# Install dev dependencies
+pip install -e ".[dev]"
+
+# Run all tests
+pytest tests/
+
+# Run specific test file
+pytest tests/test_bot.py -v
+
+# Run specific test
+pytest tests/test_bot.py::TestIsAllowed::test_allowlist_accepts_known_user -v
+```
+
+All tests should pass with these changes as the Settings class:
+1. Accepts the new nested structure from `make_settings()`
+2. Provides backward-compatible properties for old code