Switch config format from YAML to TOML

Replaces gopkg.in/yaml.v3 with github.com/BurntSushi/toml. Updates
struct tags, default config path, Dockerfile CMD, README, and ships
config.example.toml in place of the YAML example. ${ENV_VAR}
expansion still runs on the raw file before parsing, so the behavior
is unchanged.

README.md

@@ -19,7 +19,7 @@ Any member of an allowlisted room may issue commands. The bot auto-joins on invi
 
 ## Configuration
 
-Copy `config.example.yaml` to `config.yaml` and edit. The loader expands `${ENV_VAR}` references at load time, so secrets can come from the environment.
+Copy `config.example.toml` to `config.toml` and edit. The loader expands `${ENV_VAR}` references at load time, so secrets can come from the environment.
 
 Required:
 
@@ -43,7 +43,7 @@ curl -H "X-Api-Key: YOUR_KEY" https://radarr.example.com/api/v3/qualityprofile
 
 ```bash
 go build -tags goolm -o bellhop ./
-./bellhop -config config.yaml
+./bellhop -config config.toml
 ```
 
 The `goolm` tag uses the pure-Go olm implementation so libolm isn't needed.
@@ -54,7 +54,7 @@ The `goolm` tag uses the pure-Go olm implementation so libolm isn't needed.
 docker build -t bellhop .
 docker run -d \
   --name bellhop \
-  -v "$PWD/config.yaml:/app/config.yaml:ro" \
+  -v "$PWD/config.toml:/app/config.toml:ro" \
   -v bellhop-data:/app/data \
   bellhop
 ```
@@ -70,7 +70,7 @@ The bot bootstraps cross-signing on first run and persists Olm/Megolm sessions i
 ```
 main.go
 internal/
-  config/   — YAML loader with ${ENV_VAR} expansion
+  config/   — TOML loader with ${ENV_VAR} expansion
   matrix/   — mautrix client: login, device persistence, E2EE, sync loop
   arr/      — Radarr/Sonarr/Lidarr HTTP clients
   bot/      — command parser + dispatch + threaded replies