fix(cli): include room description in debug config output

[sanity]

Problem

riverctl debug config <OWNER> — the command literally named "Show room configuration" — printed the room name and every limit but silently dropped the description. The only way to see a room's description from the CLI was riverctl room config <OWNER> with no flags, which is non-obvious. Ian hit this gap while fixing the Freenet Official room's description (a broken Dashboard link in the room header).

Approach

Add description: Option<String> to the RoomConfig view and print it in both the human and JSON branches of debug config ("(none)" / null when unset).

The construction is extracted into a small testable RoomConfig::from_configuration constructor (previously an inline struct literal in the handler). The description is rendered lossily and not unsealed for private rooms — this matches the existing no-flags room config display and keeps debug config a pure read-only view.

Scope kept deliberately narrow: only debug config is touched. debug room-state remains a counts summary.

Testing

• room_config_includes_description — pins that a set description survives both the struct and the JSON projection.
• room_config_description_none_when_unset — pins that an unset description serializes as null (and renders "(none)" via unwrap_or).
• cargo test -p riverctl --lib debug:: green (4 passed).
• cargo clippy -p riverctl clean.
• Verified end-to-end against a live node: both debug config and debug config -f json now show the description.

[AI-assisted - Claude]