feat: migrate config to pydantic-settings (#65)

docs/architecture.md

@@ -274,7 +274,8 @@ flowchart LR
 
     subgraph toml_contents["takopi.toml"]
         direction TB
-        global["bot_token<br/>chat_id<br/>default_engine"]
+        global["transport<br/>default_engine"]
+        telegram_cfg["[transports.telegram]<br/>bot_token = ...<br/>chat_id = ..."]
         claude_cfg["[claude]<br/>model = ..."]
         codex_cfg["[codex]<br/>model = ..."]
         projects_cfg["[projects.alias]<br/>path = ...<br/>worktrees_dir = ...<br/>default_engine = ..."]

docs/developing.md

@@ -229,11 +229,18 @@ Self-documenting msgspec schemas for decoding engine JSONL streams.
 class ConfigError(RuntimeError): ...
 ```
 
-### `telegram/config.py` - Configuration loading
+### `settings.py` - Settings loading
 
 ```python
-def load_telegram_config() -> tuple[dict, Path]:
-    # Loads ~/.takopi/takopi.toml
+def load_settings(path: str | Path | None = None) -> tuple[TakopiSettings, Path]:
+    # Loads ~/.takopi/takopi.toml (TOML + env), validates via pydantic-settings
+```
+
+### `config_store.py` - Raw TOML read/write
+
+```python
+def read_raw_toml(path: Path) -> dict:
+    # Loads TOML for merge/update without clobbering extra sections
 ```
 
 ### `logging.py` - Secure logging setup

docs/projects.md

@@ -20,6 +20,9 @@ All config lives in `~/.takopi/takopi.toml`.
 ```toml
 default_engine = "codex"       # optional
 default_project = "z80"        # optional
+transport = "telegram"         # required
+
+[transports.telegram]
 bot_token = "..."              # required
 chat_id = 123                  # required