Use semantic filenames for visual companion screens

Server now watches directory for new .html files instead of a single
screen file. Claude writes to semantically named files like
platform.html, style.html, layout.html - each screen is a new file.

Benefits:
- No need to read before write (files are always new)
- Semantic filenames describe what's on screen
- History preserved in directory for debugging
- Server serves newest file by mtime automatically

Updated: index.js, start-server.sh, and all documentation.

lib/brainstorm-server/start-server.sh

@@ -11,7 +11,6 @@ SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 # Generate unique session directory
 SESSION_ID="$$-$(date +%s)"
 SCREEN_DIR="/tmp/brainstorm-${SESSION_ID}"
-SCREEN_FILE="${SCREEN_DIR}/screen.html"
 PID_FILE="${SCREEN_DIR}/.server.pid"
 LOG_FILE="${SCREEN_DIR}/.server.log"
 
@@ -27,7 +26,7 @@ fi
 
 # Start server, capturing output to log file
 cd "$SCRIPT_DIR"
-BRAINSTORM_SCREEN="$SCREEN_FILE" node index.js > "$LOG_FILE" 2>&1 &
+BRAINSTORM_DIR="$SCREEN_DIR" node index.js > "$LOG_FILE" 2>&1 &
 SERVER_PID=$!
 echo "$SERVER_PID" > "$PID_FILE"