story-kit: merge 277 - bot uses configured display_name

Adds system_prompt parameter to chat_stream so the Matrix bot
passes "Your name is {name}" to Claude Code. Reads display_name
from bot.toml config. Resolved conflicts by integrating bot_name
into master's permission-handling code structure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/llm/chat.rs

@@ -294,6 +294,7 @@ where
                 &user_message,
                 &project_root.to_string_lossy(),
                 config.session_id.as_deref(),
+                None,
                 &mut cancel_rx,
                 |token| on_token(token),
                 |thinking| on_thinking(thinking),