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.

config.example.toml

@@ -0,0 +1,43 @@
+# Bellhop config. Values like ${ENV_VAR} are expanded from the environment
+# at load time so secrets can stay out of this file.
+
+[matrix]
+homeserver = "https://matrix.example.com"
+user_id = "@bellhop:example.com"
+password = "${BELLHOP_MATRIX_PASSWORD}"
+
+# Optional. Defaults shown.
+display_name = "Bellhop"
+data_dir = "./data"                  # device.json + crypto.db live here
+pickle_key = "${BELLHOP_PICKLE_KEY}" # encrypts the crypto store; pick something stable
+command_prefix = "!"
+
+# Rooms the bot will respond to commands in. Messages anywhere else are
+# ignored. The bot auto-joins on invite, but joining alone does not grant
+# command access — the room ID must appear here.
+allowed_rooms = [
+  "!room-id-one:example.com",
+  "!room-id-two:example.com",
+]
+
+# Configure any subset. Omit a section to disable that command:
+# leaving out [services.radarr] makes `!movie` reply "Radarr is not configured".
+
+[services.radarr]
+url = "https://radarr.example.com"
+api_key = "${RADARR_API_KEY}"
+quality_profile_id = 1
+root_folder = "/movies"
+
+[services.sonarr]
+url = "https://sonarr.example.com"
+api_key = "${SONARR_API_KEY}"
+quality_profile_id = 1
+root_folder = "/tv"
+
+[services.lidarr]
+url = "https://lidarr.example.com"
+api_key = "${LIDARR_API_KEY}"
+quality_profile_id = 1
+metadata_profile_id = 1   # required for Lidarr only
+root_folder = "/music"